diff --git a/backend/src/routes/chat.ts b/backend/src/routes/chat.ts
new file mode 100644
index 0000000..e4f6009
--- /dev/null
+++ b/backend/src/routes/chat.ts
@@ -0,0 +1,147 @@
+import type { FastifyInstance } from "fastify";
+import { z } from "zod";
+import { and, eq } from "drizzle-orm";
+import { db } from "../db/index.js";
+import { chatRooms, members, teamMembers, teams } from "../db/schema.js";
+import { publicHomeserverUrl } from "../services/synapse.js";
+import {
+  getMatrixAccessToken,
+  invalidateMatrixAccessToken,
+} from "../services/matrix-token-cache.js";
+import { logAudit } from "../lib/audit.js";
+
+/**
+ * PROJ-11: minimal backend surface for the chat feature.
+ * The browser talks to Synapse directly for /sync, sending and read receipts —
+ * these endpoints only hand out a short-lived per-member Matrix access token and
+ * resolve the Matrix room id of a team.
+ */
+
+const roomQuerySchema = z.object({
+  teamId: z.string().uuid(),
+});
+
+/**
+ * `refresh: true` is sent by the frontend's single automatic retry after Synapse
+ * rejected a token — otherwise it would just get the same cached token back.
+ */
+const tokenBodySchema = z.object({
+  refresh: z.boolean().optional(),
+});
+
+export async function chatRoutes(app: FastifyInstance) {
+  app.post(
+    "/api/chat/token",
+    {
+      preHandler: app.authenticate,
+      config: { rateLimit: { max: 10, timeWindow: "1 minute" } },
+    },
+    async (request, reply) => {
+      const parsedBody = tokenBodySchema.safeParse(request.body ?? {});
+      if (!parsedBody.success) {
+        return reply.code(400).send({ error: parsedBody.error.issues[0].message });
+      }
+
+      const rows = await db
+        .select({
+          id: members.id,
+          organizationId: members.organizationId,
+          matrixUserId: members.matrixUserId,
+        })
+        .from(members)
+        .where(and(eq(members.userId, request.user.sub), eq(members.status, "active")));
+
+      const provisioned = rows.find((row) => row.matrixUserId);
+      if (rows.length === 0) {
+        return reply.code(403).send({ error: "Forbidden" });
+      }
+      if (!provisioned?.matrixUserId) {
+        // Provisioning has not run yet — the UI shows "Chat wird eingerichtet" and polls.
+        return reply.code(409).send({ error: "matrix_not_provisioned" });
+      }
+      const matrixUserId = provisioned.matrixUserId;
+
+      if (parsedBody.data.refresh) {
+        invalidateMatrixAccessToken(matrixUserId);
+      }
+
+      try {
+        const { accessToken, expiresInMs, minted } = await getMatrixAccessToken(matrixUserId);
+
+        if (minted) {
+          // Only newly minted tokens are audited — cache hits would flood the log.
+          // The token itself is never written to the audit log.
+          await logAudit({
+            organizationId: provisioned.organizationId,
+            actorUserId: request.user.sub,
+            action: "chat.token_issued",
+            targetType: "member",
+            targetId: provisioned.id,
+            metadata: {
+              matrixUserId,
+              expiresInMs,
+              refresh: parsedBody.data.refresh === true,
+            },
+          });
+        }
+
+        return reply.send({
+          homeserverUrl: publicHomeserverUrl(),
+          userId: matrixUserId,
+          accessToken,
+          expiresInMs,
+        });
+      } catch (err) {
+        request.log.error({ err }, "[chat] failed to mint matrix access token");
+        return reply.code(502).send({ error: "synapse_unavailable" });
+      }
+    },
+  );
+
+  app.get("/api/chat/room", { preHandler: app.authenticate }, async (request, reply) => {
+    const parsed = roomQuerySchema.safeParse(request.query);
+    if (!parsed.success) {
+      return reply.code(400).send({ error: parsed.error.issues[0].message });
+    }
+    const { teamId } = parsed.data;
+
+    const [team] = await db
+      .select({ organizationId: teams.organizationId })
+      .from(teams)
+      .where(eq(teams.id, teamId))
+      .limit(1);
+    if (!team) {
+      return reply.code(404).send({ error: "Team not found" });
+    }
+
+    // Only members of that very team may read its room id.
+    const [membership] = await db
+      .select({ id: teamMembers.id })
+      .from(teamMembers)
+      .innerJoin(members, eq(members.id, teamMembers.memberId))
+      .where(
+        and(
+          eq(teamMembers.teamId, teamId),
+          eq(members.userId, request.user.sub),
+          eq(members.status, "active"),
+        ),
+      )
+      .limit(1);
+    if (!membership) {
+      return reply.code(403).send({ error: "Forbidden" });
+    }
+
+    const [room] = await db
+      .select({ matrixRoomId: chatRooms.matrixRoomId })
+      .from(chatRooms)
+      .where(and(eq(chatRooms.teamId, teamId), eq(chatRooms.scope, "team")))
+      .limit(1);
+
+    if (!room) {
+      // Room creation job has not run yet — UI shows "Raum wird eingerichtet".
+      return reply.code(409).send({ error: "room_not_provisioned" });
+    }
+
+    return reply.send({ matrixRoomId: room.matrixRoomId });
+  });
+}
diff --git a/backend/src/routes/members.ts b/backend/src/routes/members.ts
index 0468ed1..2a8c0c8 100644
--- a/backend/src/routes/members.ts
+++ b/backend/src/routes/members.ts
@@ -5,6 +5,7 @@ import { db } from "../db/index.js";
 import { members, memberStatusEnum } from "../db/schema.js";
 import { requesterIsOrganizationMember, requesterIsOrganizationAdmin } from "../lib/authz.js";
 import { deactivateUser } from "../services/synapse.js";
+import { invalidateMatrixAccessToken } from "../services/matrix-token-cache.js";
 import { logAudit } from "../lib/audit.js";
 
 const listQuerySchema = z.object({
@@ -88,6 +89,8 @@ export async function memberRoutes(app: FastifyInstance) {
       });
 
       if ((status === "suspended" || status === "left") && member.matrixUserId) {
+        // Drop any cached chat token so a suspended member cannot keep using it (PROJ-11).
+        invalidateMatrixAccessToken(member.matrixUserId);
         try {
           await deactivateUser(member.matrixUserId, false);
         } catch (err) {
diff --git a/backend/src/routes/organizations.ts b/backend/src/routes/organizations.ts
index 61cea27..74077ee 100644
--- a/backend/src/routes/organizations.ts
+++ b/backend/src/routes/organizations.ts
@@ -4,6 +4,7 @@ import { and, eq } from "drizzle-orm";
 import { db } from "../db/index.js";
 import { organizations, members, organizationRoles, users } from "../db/schema.js";
 import { logAudit } from "../lib/audit.js";
+import { enqueueSyncJob } from "../services/sync-worker.js";
 
 const createOrganizationSchema = z.object({
   name: z.string().min(2),
@@ -49,13 +50,16 @@ export async function organizationRoutes(app: FastifyInstance) {
 
     const [organization] = await db.insert(organizations).values({ name, slug }).returning();
 
+    const firstName = requester.email.split("@")[0];
+    const lastName = "";
+
     const [member] = await db
       .insert(members)
       .values({
         organizationId: organization.id,
         userId: request.user.sub,
-        firstName: requester.email.split("@")[0],
-        lastName: "",
+        firstName,
+        lastName,
         email: requester.email,
       })
       .returning({ id: members.id });
@@ -66,6 +70,15 @@ export async function organizationRoutes(app: FastifyInstance) {
       role: "ORG_ADMIN",
     });
 
+    // The org creator only ever goes through this route, never the invitation-accept
+    // path that normally triggers provisioning — without this they'd never get a
+    // Matrix account and PROJ-11 chat would poll "wird eingerichtet" forever.
+    await enqueueSyncJob(organization.id, "provision_matrix_user", {
+      memberId: member.id,
+      firstName,
+      lastName,
+    });
+
     await logAudit({
       organizationId: organization.id,
       actorUserId: request.user.sub,
diff --git a/backend/src/server.ts b/backend/src/server.ts
index a2664f8..1e5ffd2 100644
--- a/backend/src/server.ts
+++ b/backend/src/server.ts
@@ -15,6 +15,7 @@ import { departmentRoutes } from "./routes/departments.js";
 import { seasonRoutes } from "./routes/seasons.js";
 import { roleRoutes } from "./routes/roles.js";
 import { auditRoutes } from "./routes/audit.js";
+import { chatRoutes } from "./routes/chat.js";
 import { startSyncWorker, startReconciliationWorker } from "./services/sync-worker.js";
 
 const app = Fastify({ logger: true });
@@ -43,6 +44,7 @@ await app.register(departmentRoutes);
 await app.register(seasonRoutes);
 await app.register(roleRoutes);
 await app.register(auditRoutes);
+await app.register(chatRoutes);
 
 const port = Number(process.env.PORT ?? 4000);
 app.listen({ port, host: "0.0.0.0" }).catch((err) => {
diff --git a/backend/src/services/matrix-token-cache.ts b/backend/src/services/matrix-token-cache.ts
new file mode 100644
index 0000000..a46e3b8
--- /dev/null
+++ b/backend/src/services/matrix-token-cache.ts
@@ -0,0 +1,87 @@
+import { loginAsUser } from "./synapse.js";
+
+/**
+ * PROJ-11: server-side cache for per-member Matrix access tokens.
+ *
+ * Without it every chat tab open mints a fresh token via the Synapse admin API,
+ * which both hammers Synapse and leaves a growing pile of live tokens behind.
+ * Tokens stay in process memory only — never in the TalkTime database, never in logs.
+ */
+
+export const TOKEN_TTL_MS = 60 * 60 * 1000; // 1 hour
+/** Hand out a cached token only while it is still valid for at least this long. */
+const REFRESH_MARGIN_MS = 5 * 60 * 1000; // 5 minutes
+const MAX_ENTRIES = 5_000;
+
+interface CacheEntry {
+  accessToken: string;
+  /** Absolute epoch ms at which Synapse invalidates this token. */
+  expiresAt: number;
+}
+
+const cache = new Map<string, CacheEntry>();
+/** Dedupes concurrent mints for the same user (chat tab opened twice at once). */
+const inFlight = new Map<string, Promise<CacheEntry>>();
+
+function prune(now: number): void {
+  for (const [userId, entry] of cache) {
+    if (entry.expiresAt <= now) {
+      cache.delete(userId);
+    }
+  }
+  // Hard cap so a large org cannot grow the map without bound; oldest insertions go first.
+  while (cache.size > MAX_ENTRIES) {
+    const oldest = cache.keys().next();
+    if (oldest.done) break;
+    cache.delete(oldest.value);
+  }
+}
+
+export interface CachedMatrixToken {
+  accessToken: string;
+  /** Remaining lifetime in ms — what the browser uses to schedule a refresh. */
+  expiresInMs: number;
+  /** False when the token was served from cache (used for audit-log noise reduction). */
+  minted: boolean;
+}
+
+export async function getMatrixAccessToken(matrixUserId: string): Promise<CachedMatrixToken> {
+  const now = Date.now();
+  prune(now);
+
+  const cached = cache.get(matrixUserId);
+  if (cached && cached.expiresAt - now > REFRESH_MARGIN_MS) {
+    return {
+      accessToken: cached.accessToken,
+      expiresInMs: cached.expiresAt - now,
+      minted: false,
+    };
+  }
+
+  let pending = inFlight.get(matrixUserId);
+  if (!pending) {
+    const expiresAt = now + TOKEN_TTL_MS;
+    pending = loginAsUser(matrixUserId, expiresAt)
+      .then(({ accessToken }) => {
+        const entry: CacheEntry = { accessToken, expiresAt };
+        cache.set(matrixUserId, entry);
+        return entry;
+      })
+      .finally(() => {
+        inFlight.delete(matrixUserId);
+      });
+    inFlight.set(matrixUserId, pending);
+  }
+
+  const entry = await pending;
+  return {
+    accessToken: entry.accessToken,
+    expiresInMs: Math.max(0, entry.expiresAt - Date.now()),
+    minted: true,
+  };
+}
+
+/** Drop a cached token, e.g. after the member was deactivated or the token was rejected. */
+export function invalidateMatrixAccessToken(matrixUserId: string): void {
+  cache.delete(matrixUserId);
+}
diff --git a/backend/src/services/synapse.ts b/backend/src/services/synapse.ts
index f0cc83d..e8c1246 100644
--- a/backend/src/services/synapse.ts
+++ b/backend/src/services/synapse.ts
@@ -104,6 +104,32 @@ export async function reactivateUser(userId: string, newPassword: string): Promi
   });
 }
 
+/**
+ * Public base URL browsers use to reach Synapse directly (PROJ-11).
+ * Falls back to the internal URL for single-host/dev setups.
+ */
+export function publicHomeserverUrl(): string {
+  return process.env.SYNAPSE_PUBLIC_BASE_URL ?? baseUrl;
+}
+
+/**
+ * Mints a short-lived access token for a single member via the Synapse admin API.
+ * The admin token itself never leaves the backend.
+ */
+export async function loginAsUser(
+  userId: string,
+  validUntilMs: number,
+): Promise<{ accessToken: string }> {
+  const result = await synapseFetch<{ access_token: string }>(
+    `/_synapse/admin/v1/users/${encodeURIComponent(userId)}/login`,
+    {
+      method: "POST",
+      body: JSON.stringify({ valid_until_ms: validUntilMs }),
+    },
+  );
+  return { accessToken: result.access_token };
+}
+
 export interface CreateRoomResult {
   roomId: string;
 }
@@ -118,6 +144,14 @@ export async function createRoom(params: {
       name: params.name,
       room_alias_name: params.alias,
       preset: "private_chat",
+      // PROJ-11: new members must see the full backlog when they join the team room.
+      initial_state: [
+        {
+          type: "m.room.history_visibility",
+          state_key: "",
+          content: { history_visibility: "shared" },
+        },
+      ],
     }),
   });
   return { roomId: result.room_id };
diff --git a/features/PROJ-11-chat.md b/features/PROJ-11-chat.md
index 79e703e..858d544 100644
--- a/features/PROJ-11-chat.md
+++ b/features/PROJ-11-chat.md
@@ -1,6 +1,6 @@
 # PROJ-11: Chat (Matrix Client-Server API)
 
-**Status:** Planned
+**Status:** In Progress
 **Created:** 2026-09-11
 
 ## Summary
@@ -45,6 +45,33 @@ Eigene Chat-Oberfläche für Team-Räume, die bereits über [[PROJ-9]] (Synapse-
 ## Notes
 Matrix-Access-Token-Handling ist sicherheitsrelevant (Token-Mapping TalkTime-User → Matrix-User) — Architektur-Skill sollte hier besonders auf Token-Lifetime, Caching-Ort (nicht im Frontend!) und Re-Auth-Flow eingehen. `backend-security-coder`/`frontend-security-coder`-Subagenten bei der Umsetzung hinzuziehen.
 
+## Implementation Notes (Frontend-Pass, 2026-09-11)
+
+Gebaut:
+- `frontend/src/lib/matrix.ts` — Token-/Raum-Abruf beim TalkTime-Backend, `createMatrixClient`, Datums-/Zeit-Formatierung, `ChatSetupPendingError`
+- `frontend/src/hooks/use-team-chat.ts` — matrix-js-sdk-Client pro geöffnetem Team, `/sync` direkt gegen Synapse, Timeline/Receipt-Listener, Backfill via `paginateEventTimeline`, Read Receipts, ein automatischer Retry mit frischem Token beim Senden
+- `frontend/src/components/chat/` — `TeamDetail.tsx` (Tabs Übersicht/Chat + Ungelesen-Badge am Tab), `TeamChat.tsx` (Verbindungs-/Setup-Zustände), `MessageList.tsx` (Tages-Gruppen, „gelesen von: …", Ältere-Nachrichten-Button), `MessageComposer.tsx`
+- `frontend/src/pages/TeamsPage.tsx` — Team-Kacheln sind jetzt auswählbar, öffnen die Detailansicht; `TeamDetail` wird lazy geladen (matrix-js-sdk landet in eigenem Chunk, nicht im Initial-Bundle)
+- Neu: `matrix-js-sdk` in `frontend/package.json` (frontend-only, wie im Tech Design vorgesehen)
+
+Backend (bewusst minimal, nicht der volle Backend-Pass):
+- `backend/src/routes/chat.ts` — `POST /api/chat/token` (mintet 1h-Token pro Mitglied, 409 `matrix_not_provisioned` wenn `matrixUserId` fehlt) und `GET /api/chat/room?teamId=` (409 `room_not_provisioned`); registriert in `server.ts`
+- `backend/src/services/synapse.ts` — `loginAsUser()` (Admin-API `/_synapse/admin/v1/users/{id}/login`) und `publicHomeserverUrl()`
+
+Offen für den `/backend`-Pass:
+- Ungelesen-Zähler auf den Team-Kacheln (außerhalb des geöffneten Chats) — bräuchte einen app-weiten Matrix-Client statt einen pro geöffnetem Team; aktuell nur Badge am Chat-Tab
+- Deployment-Voraussetzung unverändert offen: Synapse muss vom Browser aus per TLS erreichbar sein
+
+## Implementation Notes (Backend-Pass, 2026-09-11)
+
+- `backend/src/services/matrix-token-cache.ts` (neu) — In-Memory-Cache für die pro Mitglied gemin­teten Matrix-Access-Tokens (TTL 1 h, Wiederverwendung bis 5 min vor Ablauf, Dedupe gleichzeitiger Mints, harte Obergrenze 5.000 Einträge). Tokens liegen ausschließlich im Prozessspeicher, nie in der DB, nie im Log.
+- `backend/src/routes/chat.ts` — `POST /api/chat/token` nutzt den Cache; optionaler Body `{ refresh: true }` (Zod-validiert) invalidiert den Cache-Eintrag, damit der automatische Retry des Frontends nicht denselben abgelehnten Token zurückbekommt. Neu gemintete Tokens erzeugen einen Audit-Log-Eintrag `chat.token_issued` (Metadaten: matrixUserId, expiresInMs, refresh) — Cache-Treffer werden bewusst nicht geloggt, und der Token selbst steht nie im Audit-Log.
+- `backend/src/routes/members.ts` — bei Statuswechsel auf `suspended`/`left` wird der gecachte Chat-Token sofort invalidiert.
+- `backend/src/services/synapse.ts` — `createRoom()` setzt jetzt `initial_state` mit `m.room.history_visibility: "shared"`, damit neue Raum-Mitglieder die volle bisherige Historie sehen (Akzeptanzkriterium). Gilt für neu angelegte Räume; bereits bestehende Testräume müssten einmalig nachgezogen werden.
+- `frontend/src/lib/matrix.ts` / `use-team-chat.ts` — `fetchChatCredentials(token, { refresh })`, Retry beim Senden fordert explizit einen frischen Token an.
+
+Weiterhin offen (Berechtigung fehlte): `SYNAPSE_PUBLIC_BASE_URL` ist in `backend/.env.example` zu ergänzen (Fallback im Code ist `SYNAPSE_BASE_URL`) — die Datei ist für den Agenten nicht schreibbar, bitte manuell eintragen.
+
 ## Tech Design (Solution Architect)
 
 ### A) Komponenten-Struktur
diff --git a/features/PROJ-2-verein-und-vereinsadministrator.md b/features/PROJ-2-verein-und-vereinsadministrator.md
index 30127e5..7c69913 100644
--- a/features/PROJ-2-verein-und-vereinsadministrator.md
+++ b/features/PROJ-2-verein-und-vereinsadministrator.md
@@ -11,11 +11,14 @@ Anlage einer Vereins-Organisation (Tenant) samt Vereinsadministrator (CLUB_ADMIN
 - `frontend/src/components/dashboard/CreateOrganizationForm.tsx`
 - `frontend/src/components/dashboard/OrganizationSwitcher.tsx`
 - `frontend/src/lib/organization-context.tsx`
+- **Nachtrag 2026-09-11 (Bugfix während PROJ-11-Test gefunden):** `POST /api/organizations` provisioniert jetzt auch für den Org-Ersteller einen Matrix-Account (`enqueueSyncJob(..., "provision_matrix_user", ...)`). Vorher lief Matrix-Provisioning nur über den Einladungs-Annahme-Flow ([[PROJ-7]]) — der Ersteller/erste CLUB_ADMIN bekam nie einen Matrix-Account und hätte in [[PROJ-11]] (Chat) dauerhaft nur "wird eingerichtet" gesehen.
 
 ## Acceptance Criteria
 - [x] Nutzer kann neuen Verein anlegen und wird automatisch CLUB_ADMIN
 - [x] Multi-Tenant: ein Nutzer kann Mitglied mehrerer Vereine sein, Frontend erlaubt Wechsel
 - [x] Tenant-Isolation in Route-Handlern geprüft (kein DB-RLS-Backstop, siehe `.claude/rules/security.md`)
+- [x] Org-Ersteller bekommt automatisch einen Matrix-Account provisioniert (nicht nur eingeladene Mitglieder)
+- [x] `GET /api/organizations/mine` listet nur Organisationen mit `status = "active"` (siehe [[PROJ-3]]-Nachtrag zur Zugriffssperre bei Austritt)
 
 ## Open Items
 - Dedizierte Tenant-Isolation-Sicherheitstests (PRD-Erfolgsmetrik: 0 Cross-Club-Vorfälle) noch nicht als QA-Suite gebaut
diff --git a/frontend/package-lock.json b/frontend/package-lock.json
index daf90e2..ec8b995 100644
--- a/frontend/package-lock.json
+++ b/frontend/package-lock.json
@@ -1,11 +1,11 @@
 {
-  "name": "ai-coding-starter-kit-frontend",
+  "name": "talktime-frontend",
   "version": "1.0.0",
   "lockfileVersion": 3,
   "requires": true,
   "packages": {
     "": {
-      "name": "ai-coding-starter-kit-frontend",
+      "name": "talktime-frontend",
       "version": "1.0.0",
       "dependencies": {
         "@hookform/resolvers": "^5.2.2",
@@ -33,6 +33,7 @@
         "clsx": "^2.1.0",
         "cmdk": "^1.1.1",
         "lucide-react": "^0.562.0",
+        "matrix-js-sdk": "^42.3.0",
         "next-themes": "^0.4.6",
         "react": "^19.0.0",
         "react-dom": "^19.0.0",
@@ -302,6 +303,12 @@
         "@babel/core": "^7.0.0-0"
       }
     },
+    "node_modules/@babel/runtime": {
+      "version": "8.0.5",
+      "resolved": "https://registry.npmjs.org/@babel/runtime/-/runtime-8.0.5.tgz",
+      "integrity": "sha512-7NK+Lz3spQ52XsUGTxIEVU4jYN2/dIaX8sTxRFAUtYmttYZnVh3aehiihv+/Gb7+duMNHl2OrFcBQRyOHScxpg==",
+      "license": "MIT"
+    },
     "node_modules/@babel/template": {
       "version": "7.29.7",
       "resolved": "https://registry.npmjs.org/@babel/template/-/template-7.29.7.tgz",
@@ -1224,6 +1231,15 @@
         "@jridgewell/sourcemap-codec": "^1.4.14"
       }
     },
+    "node_modules/@matrix-org/matrix-sdk-crypto-wasm": {
+      "version": "18.8.0",
+      "resolved": "https://registry.npmjs.org/@matrix-org/matrix-sdk-crypto-wasm/-/matrix-sdk-crypto-wasm-18.8.0.tgz",
+      "integrity": "sha512-J+bYzUqtkYSrIUbDT8Razf9F1gzlpC94qeC5LQlr9QmjrzyaErwlV9v7S+gtSOeDvJCdfP8ebEDaANHiVLCYQg==",
+      "license": "Apache-2.0",
+      "engines": {
+        "node": ">= 18"
+      }
+    },
     "node_modules/@napi-rs/lzma-linux-x64-gnu": {
       "version": "1.5.1",
       "resolved": "https://registry.npmjs.org/@napi-rs/lzma-linux-x64-gnu/-/lzma-linux-x64-gnu-1.5.1.tgz",
@@ -2828,6 +2844,12 @@
       "dev": true,
       "license": "MIT"
     },
+    "node_modules/@types/events": {
+      "version": "3.0.3",
+      "resolved": "https://registry.npmjs.org/@types/events/-/events-3.0.3.tgz",
+      "integrity": "sha512-trOc4AAUThEz9hapPtSd7wf5tiQKvTtu5b371UxXdTuqzIh0ArcRspRP0i0Viu+LXstIQ1z96t1nsPxT9ol01g==",
+      "license": "MIT"
+    },
     "node_modules/@types/json-schema": {
       "version": "7.0.15",
       "resolved": "https://registry.npmjs.org/@types/json-schema/-/json-schema-7.0.15.tgz",
@@ -2927,6 +2949,12 @@
         "url": "https://github.com/sponsors/epoberezkin"
       }
     },
+    "node_modules/another-json": {
+      "version": "0.2.0",
+      "resolved": "https://registry.npmjs.org/another-json/-/another-json-0.2.0.tgz",
+      "integrity": "sha512-/Ndrl68UQLhnCdsAzEXLMFuOR546o2qbYRqCglaNHbjXrwG1ayTcdwr3zkSGOGtGXDyR5X9nCFfnyG2AFJIsqg==",
+      "license": "Apache-2.0"
+    },
     "node_modules/ansi-styles": {
       "version": "4.3.0",
       "resolved": "https://registry.npmjs.org/ansi-styles/-/ansi-styles-4.3.0.tgz",
@@ -3034,6 +3062,12 @@
       "dev": true,
       "license": "MIT"
     },
+    "node_modules/base-x": {
+      "version": "5.0.1",
+      "resolved": "https://registry.npmjs.org/base-x/-/base-x-5.0.1.tgz",
+      "integrity": "sha512-M7uio8Zt++eg3jPj+rHMfCC+IuygQHHCOU+IYsVtik6FWjuYpVt/+MRKcgsAMHh8mMFAwnB+Bs+mTrFiXjMzKg==",
+      "license": "MIT"
+    },
     "node_modules/baseline-browser-mapping": {
       "version": "2.11.21",
       "resolved": "https://registry.npmjs.org/baseline-browser-mapping/-/baseline-browser-mapping-2.11.21.tgz",
@@ -3118,6 +3152,15 @@
         "node": "^6 || ^7 || ^8 || ^9 || ^10 || ^11 || ^12 || >=13.7"
       }
     },
+    "node_modules/bs58": {
+      "version": "6.0.0",
+      "resolved": "https://registry.npmjs.org/bs58/-/bs58-6.0.0.tgz",
+      "integrity": "sha512-PD0wEnEYg6ijszw/u8s+iI3H17cTymlrwkKhDhPZq+Sokl3AU4htyBFTjAeNAlCCmg0f53g6ih3jATyCKftTfw==",
+      "license": "MIT",
+      "dependencies": {
+        "base-x": "^5.0.0"
+      }
+    },
     "node_modules/callsites": {
       "version": "3.1.0",
       "resolved": "https://registry.npmjs.org/callsites/-/callsites-3.1.0.tgz",
@@ -3288,6 +3331,19 @@
       "dev": true,
       "license": "MIT"
     },
+    "node_modules/content-type": {
+      "version": "2.1.0",
+      "resolved": "https://registry.npmjs.org/content-type/-/content-type-2.1.0.tgz",
+      "integrity": "sha512-mj7UPXE0jaqaOsukNZRUEfEi2AcL7C/vwmwcHV0O97eO1E1pxBZuyjlZrx5seTaNBg1U6+o35wpa35Qfcc+7ag==",
+      "license": "MIT",
+      "engines": {
+        "node": ">=18"
+      },
+      "funding": {
+        "type": "opencollective",
+        "url": "https://opencollective.com/express"
+      }
+    },
     "node_modules/convert-source-map": {
       "version": "2.0.0",
       "resolved": "https://registry.npmjs.org/convert-source-map/-/convert-source-map-2.0.0.tgz",
@@ -3649,6 +3705,15 @@
         "node": ">=0.10.0"
       }
     },
+    "node_modules/events": {
+      "version": "3.3.0",
+      "resolved": "https://registry.npmjs.org/events/-/events-3.3.0.tgz",
+      "integrity": "sha512-mQw+2fkQbALzQ7V0MY0IqdnXNOeTtP4r0lN9z7AAawCXgqea7bDii20AYrIBrFd/Hx0M2Ocz6S111CaFkUcb0Q==",
+      "license": "MIT",
+      "engines": {
+        "node": ">=0.8.x"
+      }
+    },
     "node_modules/fast-deep-equal": {
       "version": "3.1.3",
       "resolved": "https://registry.npmjs.org/fast-deep-equal/-/fast-deep-equal-3.1.3.tgz",
@@ -3988,6 +4053,18 @@
         "node": ">=0.10.0"
       }
     },
+    "node_modules/is-network-error": {
+      "version": "1.3.2",
+      "resolved": "https://registry.npmjs.org/is-network-error/-/is-network-error-1.3.2.tgz",
+      "integrity": "sha512-PhBY86zaxNZUuWP6h13Vu5oFe0XY6/UlKzQnYFELzGVHygP3MxmvTfYSG7GN3aIab/iWudSMgjSnG9Dq+nHrgA==",
+      "license": "MIT",
+      "engines": {
+        "node": ">=16"
+      },
+      "funding": {
+        "url": "https://github.com/sponsors/sindresorhus"
+      }
+    },
     "node_modules/is-number": {
       "version": "7.0.0",
       "resolved": "https://registry.npmjs.org/is-number/-/is-number-7.0.0.tgz",
@@ -4160,6 +4237,19 @@
       "dev": true,
       "license": "MIT"
     },
+    "node_modules/loglevel": {
+      "version": "1.9.2",
+      "resolved": "https://registry.npmjs.org/loglevel/-/loglevel-1.9.2.tgz",
+      "integrity": "sha512-HgMmCqIJSAKqo68l0rS2AanEWfkxaZ5wNiEFb5ggm08lDs9Xl2KxBlX3PTcaD2chBM1gXAYf491/M2Rv8Jwayg==",
+      "license": "MIT",
+      "engines": {
+        "node": ">= 0.6.0"
+      },
+      "funding": {
+        "type": "tidelift",
+        "url": "https://tidelift.com/funding/github/npm/loglevel"
+      }
+    },
     "node_modules/lru-cache": {
       "version": "5.1.1",
       "resolved": "https://registry.npmjs.org/lru-cache/-/lru-cache-5.1.1.tgz",
@@ -4179,6 +4269,44 @@
         "react": "^16.5.1 || ^17.0.0 || ^18.0.0 || ^19.0.0"
       }
     },
+    "node_modules/matrix-events-sdk": {
+      "version": "0.0.1",
+      "resolved": "https://registry.npmjs.org/matrix-events-sdk/-/matrix-events-sdk-0.0.1.tgz",
+      "integrity": "sha512-1QEOsXO+bhyCroIe2/A5OwaxHvBm7EsSQ46DEDn8RBIfQwN5HWBpFvyWWR4QY0KHPPnnJdI99wgRiAl7Ad5qaA==",
+      "license": "Apache-2.0"
+    },
+    "node_modules/matrix-js-sdk": {
+      "version": "42.3.0",
+      "resolved": "https://registry.npmjs.org/matrix-js-sdk/-/matrix-js-sdk-42.3.0.tgz",
+      "integrity": "sha512-jXXFLlaA25zQ2oxXCgbAEubGC8KpF8OabSf387W0RXfkwqhNXEbU+veeUaZMT+bCbyE5nDxkN3iBy038VbntQg==",
+      "license": "Apache-2.0",
+      "dependencies": {
+        "@babel/runtime": "^8.0.0",
+        "@matrix-org/matrix-sdk-crypto-wasm": "^18.4.0",
+        "another-json": "^0.2.0",
+        "bs58": "^6.0.0",
+        "content-type": "^2.0.0",
+        "loglevel": "^1.9.2",
+        "matrix-events-sdk": "0.0.1",
+        "matrix-widget-api": "^1.18.0",
+        "p-retry": "8",
+        "sdp-transform": "^3.0.0",
+        "unhomoglyph": "^1.0.6"
+      },
+      "engines": {
+        "node": ">=22.0.0"
+      }
+    },
+    "node_modules/matrix-widget-api": {
+      "version": "1.19.0",
+      "resolved": "https://registry.npmjs.org/matrix-widget-api/-/matrix-widget-api-1.19.0.tgz",
+      "integrity": "sha512-Qe7L6G1mQJphBC47/Bzp5ljPIK1UARgLiuGgS+/KmEs/eaNHkQ3+ArD0/0kvMV+d6kCGAZ9OKRqsnuPjizI+tw==",
+      "license": "Apache-2.0",
+      "dependencies": {
+        "@types/events": "^3.0.0",
+        "events": "^3.2.0"
+      }
+    },
     "node_modules/merge2": {
       "version": "1.4.1",
       "resolved": "https://registry.npmjs.org/merge2/-/merge2-1.4.1.tgz",
@@ -4361,6 +4489,21 @@
         "url": "https://github.com/sponsors/sindresorhus"
       }
     },
+    "node_modules/p-retry": {
+      "version": "8.0.1",
+      "resolved": "https://registry.npmjs.org/p-retry/-/p-retry-8.0.1.tgz",
+      "integrity": "sha512-NAigyu8hfvJe7MsS18mnc4is0MZtau3QMdBklaJKK23oBQb6occO3UPM3vHmTckW8KhfyjMZaUOqdTFEYliHgg==",
+      "license": "MIT",
+      "dependencies": {
+        "is-network-error": "^1.3.0"
+      },
+      "engines": {
+        "node": ">=22"
+      },
+      "funding": {
+        "url": "https://github.com/sponsors/sindresorhus"
+      }
+    },
     "node_modules/parent-module": {
       "version": "1.0.1",
       "resolved": "https://registry.npmjs.org/parent-module/-/parent-module-1.0.1.tgz",
@@ -4939,6 +5082,15 @@
       "integrity": "sha512-eNv+WrVbKu1f3vbYJT/xtiF5syA5HPIMtf9IgY/nKg0sWqzAUEvqY/xm7OcZc/qafLx/iO9FgOmeSAp4v5ti/Q==",
       "license": "MIT"
     },
+    "node_modules/sdp-transform": {
+      "version": "3.0.0",
+      "resolved": "https://registry.npmjs.org/sdp-transform/-/sdp-transform-3.0.0.tgz",
+      "integrity": "sha512-gfYVRGxjHkGF2NPeUWHw5u6T/KGFtS5/drPms73gaSuMaVHKCY3lpLnGDfswVQO0kddeePoti09AwhYP4zA8dQ==",
+      "license": "MIT",
+      "bin": {
+        "sdp-verify": "checker.js"
+      }
+    },
     "node_modules/semver": {
       "version": "6.3.1",
       "resolved": "https://registry.npmjs.org/semver/-/semver-6.3.1.tgz",
@@ -5245,6 +5397,12 @@
       "dev": true,
       "license": "MIT"
     },
+    "node_modules/unhomoglyph": {
+      "version": "1.0.6",
+      "resolved": "https://registry.npmjs.org/unhomoglyph/-/unhomoglyph-1.0.6.tgz",
+      "integrity": "sha512-7uvcWI3hWshSADBu4JpnyYbTVc7YlhF5GDW/oPD5AxIxl34k4wXR3WDkPnzLxkN32LiTCTKMQLtKVZiwki3zGg==",
+      "license": "MIT"
+    },
     "node_modules/update-browserslist-db": {
       "version": "1.3.2",
       "resolved": "https://registry.npmjs.org/update-browserslist-db/-/update-browserslist-db-1.3.2.tgz",
diff --git a/frontend/package.json b/frontend/package.json
index 27a9b67..67b8567 100644
--- a/frontend/package.json
+++ b/frontend/package.json
@@ -35,6 +35,7 @@
     "clsx": "^2.1.0",
     "cmdk": "^1.1.1",
     "lucide-react": "^0.562.0",
+    "matrix-js-sdk": "^42.3.0",
     "next-themes": "^0.4.6",
     "react": "^19.0.0",
     "react-dom": "^19.0.0",
diff --git a/frontend/src/components/chat/MessageComposer.tsx b/frontend/src/components/chat/MessageComposer.tsx
new file mode 100644
index 0000000..bfcd0b5
--- /dev/null
+++ b/frontend/src/components/chat/MessageComposer.tsx
@@ -0,0 +1,55 @@
+import { useState, type FormEvent, type KeyboardEvent } from "react";
+import { Button } from "@/components/ui/button";
+import { Textarea } from "@/components/ui/textarea";
+
+interface MessageComposerProps {
+  disabled: boolean;
+  sending: boolean;
+  onSend: (body: string) => Promise<void>;
+}
+
+export function MessageComposer({ disabled, sending, onSend }: MessageComposerProps) {
+  const [value, setValue] = useState("");
+
+  async function submit() {
+    const body = value.trim();
+    if (!body || disabled || sending) return;
+    setValue("");
+    try {
+      await onSend(body);
+    } catch {
+      // Keep the text so nothing is lost when sending failed.
+      setValue(body);
+    }
+  }
+
+  function onKeyDown(event: KeyboardEvent<HTMLTextAreaElement>) {
+    if (event.key === "Enter" && !event.shiftKey) {
+      event.preventDefault();
+      void submit();
+    }
+  }
+
+  function onSubmit(event: FormEvent) {
+    event.preventDefault();
+    void submit();
+  }
+
+  return (
+    <form onSubmit={onSubmit} className="flex items-end gap-2 border-t p-3">
+      <Textarea
+        value={value}
+        onChange={(e) => setValue(e.target.value)}
+        onKeyDown={onKeyDown}
+        disabled={disabled}
+        rows={1}
+        placeholder="Nachricht schreiben…"
+        aria-label="Nachricht schreiben"
+        className="max-h-32 min-h-[40px] resize-none"
+      />
+      <Button type="submit" disabled={disabled || sending || !value.trim()}>
+        {sending ? "Sendet…" : "Senden"}
+      </Button>
+    </form>
+  );
+}
diff --git a/frontend/src/components/chat/MessageList.tsx b/frontend/src/components/chat/MessageList.tsx
new file mode 100644
index 0000000..fb501ed
--- /dev/null
+++ b/frontend/src/components/chat/MessageList.tsx
@@ -0,0 +1,133 @@
+import { useEffect, useMemo, useRef } from "react";
+import { Button } from "@/components/ui/button";
+import { ScrollArea } from "@/components/ui/scroll-area";
+import { Skeleton } from "@/components/ui/skeleton";
+import { cn } from "@/lib/utils";
+import { formatDayLabel, formatTime } from "@/lib/matrix";
+import type { ChatMessage } from "@/hooks/use-team-chat";
+
+interface MessageListProps {
+  messages: ChatMessage[];
+  ownUserId: string | null;
+  loading: boolean;
+  canLoadMore: boolean;
+  loadingMore: boolean;
+  onLoadMore: () => void;
+}
+
+interface DayGroup {
+  key: string;
+  label: string;
+  messages: ChatMessage[];
+}
+
+function groupByDay(messages: ChatMessage[]): DayGroup[] {
+  const groups: DayGroup[] = [];
+  for (const message of messages) {
+    const date = new Date(message.timestamp);
+    const key = date.toDateString();
+    const last = groups[groups.length - 1];
+    if (last && last.key === key) {
+      last.messages.push(message);
+    } else {
+      groups.push({ key, label: formatDayLabel(date), messages: [message] });
+    }
+  }
+  return groups;
+}
+
+export function MessageList({
+  messages,
+  ownUserId,
+  loading,
+  canLoadMore,
+  loadingMore,
+  onLoadMore,
+}: MessageListProps) {
+  const bottomRef = useRef<HTMLDivElement | null>(null);
+  const groups = useMemo(() => groupByDay(messages), [messages]);
+  const lastId = messages[messages.length - 1]?.id;
+
+  useEffect(() => {
+    bottomRef.current?.scrollIntoView({ block: "end" });
+  }, [lastId]);
+
+  if (loading) {
+    return (
+      <div className="space-y-3 p-4" aria-busy="true" aria-label="Nachrichten werden geladen">
+        <Skeleton className="h-12 w-2/3" />
+        <Skeleton className="h-12 w-1/2" />
+        <Skeleton className="h-12 w-3/5" />
+      </div>
+    );
+  }
+
+  if (messages.length === 0) {
+    return (
+      <div className="flex h-full items-center justify-center p-6">
+        <p className="text-sm text-muted-foreground">
+          Noch keine Nachrichten. Schreib die erste.
+        </p>
+      </div>
+    );
+  }
+
+  return (
+    <ScrollArea className="h-full">
+      <div className="space-y-4 p-4" role="log" aria-live="polite" aria-label="Chatverlauf">
+        {canLoadMore && (
+          <div className="flex justify-center">
+            <Button variant="outline" size="sm" onClick={onLoadMore} disabled={loadingMore}>
+              {loadingMore ? "Lädt…" : "Ältere Nachrichten laden"}
+            </Button>
+          </div>
+        )}
+
+        {groups.map((group) => (
+          <section key={group.key} className="space-y-2">
+            <div className="flex items-center gap-3">
+              <span className="h-px flex-1 bg-border" />
+              <h3 className="text-xs font-medium text-muted-foreground">{group.label}</h3>
+              <span className="h-px flex-1 bg-border" />
+            </div>
+
+            {group.messages.map((message) => {
+              const own = message.senderId === ownUserId;
+              return (
+                <article
+                  key={message.id}
+                  className={cn("flex flex-col", own ? "items-end" : "items-start")}
+                >
+                  <div
+                    className={cn(
+                      "max-w-[85%] rounded-lg px-3 py-2 sm:max-w-[70%]",
+                      own ? "bg-primary text-primary-foreground" : "bg-muted",
+                      message.pending && "opacity-60",
+                    )}
+                  >
+                    {!own && (
+                      <p className="text-xs font-medium opacity-80">{message.senderName}</p>
+                    )}
+                    <p className="whitespace-pre-wrap break-words text-sm">{message.body}</p>
+                    <p className="mt-1 text-[10px] opacity-70">
+                      <time dateTime={new Date(message.timestamp).toISOString()}>
+                        {formatTime(new Date(message.timestamp))}
+                      </time>
+                      {message.pending && " · wird gesendet…"}
+                    </p>
+                  </div>
+                  {own && message.readBy.length > 0 && (
+                    <p className="mt-0.5 text-[10px] text-muted-foreground">
+                      gelesen von: {message.readBy.join(", ")}
+                    </p>
+                  )}
+                </article>
+              );
+            })}
+          </section>
+        ))}
+        <div ref={bottomRef} />
+      </div>
+    </ScrollArea>
+  );
+}
diff --git a/frontend/src/components/chat/TeamChat.tsx b/frontend/src/components/chat/TeamChat.tsx
new file mode 100644
index 0000000..1116165
--- /dev/null
+++ b/frontend/src/components/chat/TeamChat.tsx
@@ -0,0 +1,80 @@
+import { useEffect } from "react";
+import { Alert, AlertDescription, AlertTitle } from "@/components/ui/alert";
+import { Button } from "@/components/ui/button";
+import type { useTeamChat } from "@/hooks/use-team-chat";
+import { MessageComposer } from "./MessageComposer";
+import { MessageList } from "./MessageList";
+
+interface TeamChatProps {
+  /** Lifted into the parent so the unread badge can live on the tab trigger. */
+  chat: ReturnType<typeof useTeamChat>;
+  teamName: string;
+}
+
+export function TeamChat({ chat, teamName }: TeamChatProps) {
+  const { status, markRead, messages } = chat;
+  const lastId = messages[messages.length - 1]?.id;
+
+  useEffect(() => {
+    if (status === "ready" && lastId) {
+      markRead();
+    }
+  }, [status, lastId, markRead]);
+
+  if (status === "pending") {
+    const setupText =
+      chat.pendingReason === "room_not_provisioned"
+        ? "Der Gruppenraum für dieses Team wird gerade angelegt. Sobald er bereit ist, erscheint der Verlauf hier automatisch."
+        : "Dein Chat-Zugang wird gerade eingerichtet. Das dauert in der Regel nur kurz — diese Seite aktualisiert sich automatisch.";
+    return (
+      <Alert>
+        <AlertTitle>
+          {chat.pendingReason === "room_not_provisioned"
+            ? "Raum wird eingerichtet"
+            : "Chat wird eingerichtet"}
+        </AlertTitle>
+        <AlertDescription>{setupText}</AlertDescription>
+      </Alert>
+    );
+  }
+
+  if (status === "error") {
+    return (
+      <Alert variant="destructive">
+        <AlertTitle>Chat nicht verfügbar</AlertTitle>
+        <AlertDescription className="space-y-3">
+          <p>{chat.error ?? "Verbindung zum Chat-Server fehlgeschlagen."}</p>
+          <Button variant="outline" size="sm" onClick={chat.retry}>
+            Erneut versuchen
+          </Button>
+        </AlertDescription>
+      </Alert>
+    );
+  }
+
+  return (
+    <div
+      className="flex h-[60vh] flex-col rounded-lg border sm:h-[65vh]"
+      aria-label={`Chat für ${teamName}`}
+    >
+      <div className="min-h-0 flex-1">
+        <MessageList
+          messages={chat.messages}
+          ownUserId={chat.ownUserId}
+          loading={status === "loading"}
+          canLoadMore={chat.canLoadMore}
+          loadingMore={chat.loadingMore}
+          onLoadMore={() => void chat.loadMore()}
+        />
+      </div>
+      {chat.error && status === "ready" && (
+        <p className="px-3 pb-1 text-xs text-destructive">{chat.error}</p>
+      )}
+      <MessageComposer
+        disabled={status !== "ready"}
+        sending={chat.sending}
+        onSend={chat.sendMessage}
+      />
+    </div>
+  );
+}
diff --git a/frontend/src/components/chat/TeamDetail.tsx b/frontend/src/components/chat/TeamDetail.tsx
new file mode 100644
index 0000000..4279052
--- /dev/null
+++ b/frontend/src/components/chat/TeamDetail.tsx
@@ -0,0 +1,71 @@
+import { Badge } from "@/components/ui/badge";
+import { Button } from "@/components/ui/button";
+import { Card, CardContent } from "@/components/ui/card";
+import { Tabs, TabsContent, TabsList, TabsTrigger } from "@/components/ui/tabs";
+import { useAuth } from "@/lib/auth-context";
+import { useTeamChat } from "@/hooks/use-team-chat";
+import { TeamChat } from "./TeamChat";
+
+interface TeamDetailProps {
+  teamId: string;
+  teamName: string;
+  departmentName: string;
+  seasonName: string;
+  onClose: () => void;
+}
+
+export function TeamDetail({
+  teamId,
+  teamName,
+  departmentName,
+  seasonName,
+  onClose,
+}: TeamDetailProps) {
+  const { token } = useAuth();
+  const chat = useTeamChat(token, teamId);
+
+  return (
+    <Card>
+      <CardContent className="space-y-4 pt-6">
+        <div className="flex items-start justify-between gap-3">
+          <div>
+            <h2 className="text-lg font-semibold">{teamName}</h2>
+            <p className="text-xs text-muted-foreground">
+              {departmentName} · {seasonName}
+            </p>
+          </div>
+          <Button variant="ghost" size="sm" onClick={onClose}>
+            Schließen
+          </Button>
+        </div>
+
+        <Tabs defaultValue="overview">
+          <TabsList>
+            <TabsTrigger value="overview">Übersicht</TabsTrigger>
+            <TabsTrigger value="chat" className="gap-2">
+              Chat
+              {chat.unreadCount > 0 && (
+                <Badge variant="secondary" aria-label={`${chat.unreadCount} ungelesene Nachrichten`}>
+                  {chat.unreadCount}
+                </Badge>
+              )}
+            </TabsTrigger>
+          </TabsList>
+
+          <TabsContent value="overview" className="pt-4">
+            <dl className="grid grid-cols-2 gap-2 text-sm">
+              <dt className="text-muted-foreground">Abteilung</dt>
+              <dd>{departmentName}</dd>
+              <dt className="text-muted-foreground">Saison</dt>
+              <dd>{seasonName}</dd>
+            </dl>
+          </TabsContent>
+
+          <TabsContent value="chat" className="pt-4">
+            <TeamChat chat={chat} teamName={teamName} />
+          </TabsContent>
+        </Tabs>
+      </CardContent>
+    </Card>
+  );
+}
diff --git a/frontend/src/hooks/use-team-chat.ts b/frontend/src/hooks/use-team-chat.ts
new file mode 100644
index 0000000..de3e672
--- /dev/null
+++ b/frontend/src/hooks/use-team-chat.ts
@@ -0,0 +1,253 @@
+import { useCallback, useEffect, useRef, useState } from "react";
+import {
+  ClientEvent,
+  Direction,
+  EventType,
+  RoomEvent,
+  SyncState,
+  type MatrixClient,
+  type MatrixEvent,
+  type Room,
+} from "matrix-js-sdk";
+import {
+  ChatSetupPendingError,
+  createMatrixClient,
+  fetchChatCredentials,
+  fetchTeamRoomId,
+  type ChatSetupReason,
+} from "@/lib/matrix";
+
+export interface ChatMessage {
+  id: string;
+  senderId: string;
+  senderName: string;
+  body: string;
+  timestamp: number;
+  pending: boolean;
+  readBy: string[];
+}
+
+export type ChatStatus = "loading" | "pending" | "error" | "ready";
+
+interface UseTeamChatResult {
+  status: ChatStatus;
+  pendingReason: ChatSetupReason | null;
+  error: string | null;
+  messages: ChatMessage[];
+  ownUserId: string | null;
+  unreadCount: number;
+  canLoadMore: boolean;
+  loadingMore: boolean;
+  sending: boolean;
+  sendMessage: (body: string) => Promise<void>;
+  loadMore: () => Promise<void>;
+  markRead: () => void;
+  retry: () => void;
+}
+
+const POLL_INTERVAL_MS = 15_000;
+const PAGE_SIZE = 30;
+
+function readReceiptsFor(room: Room, eventId: string, ownUserId: string): string[] {
+  const event = room.findEventById(eventId);
+  if (!event) return [];
+  return room
+    .getReceiptsForEvent(event)
+    .filter((r) => r.type === "m.read" && r.userId !== ownUserId)
+    .map((r) => room.getMember(r.userId)?.name ?? r.userId);
+}
+
+function toMessages(room: Room, ownUserId: string): ChatMessage[] {
+  return room
+    .getLiveTimeline()
+    .getEvents()
+    .filter((event) => event.getType() === EventType.RoomMessage && !event.isRedacted())
+    .map((event) => {
+      const id = event.getId() ?? `${event.getTs()}`;
+      return {
+        id,
+        senderId: event.getSender() ?? "",
+        senderName: room.getMember(event.getSender() ?? "")?.name ?? event.getSender() ?? "",
+        body: (event.getContent().body as string) ?? "",
+        timestamp: event.getTs(),
+        pending: event.status !== null,
+        readBy: readReceiptsFor(room, id, ownUserId),
+      };
+    });
+}
+
+export function useTeamChat(authToken: string | null, teamId: string | null): UseTeamChatResult {
+  const [status, setStatus] = useState<ChatStatus>("loading");
+  const [pendingReason, setPendingReason] = useState<ChatSetupReason | null>(null);
+  const [error, setError] = useState<string | null>(null);
+  const [messages, setMessages] = useState<ChatMessage[]>([]);
+  const [ownUserId, setOwnUserId] = useState<string | null>(null);
+  const [unreadCount, setUnreadCount] = useState(0);
+  const [canLoadMore, setCanLoadMore] = useState(false);
+  const [loadingMore, setLoadingMore] = useState(false);
+  const [sending, setSending] = useState(false);
+  const [attempt, setAttempt] = useState(0);
+
+  const clientRef = useRef<MatrixClient | null>(null);
+  const roomRef = useRef<Room | null>(null);
+
+  const refresh = useCallback(() => {
+    const room = roomRef.current;
+    const userId = clientRef.current?.getUserId();
+    if (!room || !userId) return;
+    setMessages(toMessages(room, userId));
+    setUnreadCount(room.getUnreadNotificationCount() ?? 0);
+    setCanLoadMore(Boolean(room.getLiveTimeline().getPaginationToken(Direction.Backward)));
+  }, []);
+
+  useEffect(() => {
+    if (!authToken || !teamId) {
+      setStatus("loading");
+      return;
+    }
+
+    let cancelled = false;
+    let pollTimer: ReturnType<typeof setTimeout> | null = null;
+    let client: MatrixClient | null = null;
+
+    const onTimeline = () => !cancelled && refresh();
+
+    async function connect() {
+      setStatus("loading");
+      setError(null);
+      setPendingReason(null);
+      try {
+        const roomId = await fetchTeamRoomId(authToken!, teamId!);
+        const creds = await fetchChatCredentials(authToken!);
+        if (cancelled) return;
+
+        client = createMatrixClient(creds);
+        clientRef.current = client;
+        setOwnUserId(creds.userId);
+
+        client.on(RoomEvent.Timeline, onTimeline);
+        client.on(RoomEvent.Receipt, onTimeline);
+        client.on(RoomEvent.LocalEchoUpdated, onTimeline);
+        client.on(ClientEvent.Sync, (state: SyncState) => {
+          if (cancelled) return;
+          if (state === SyncState.Prepared || state === SyncState.Syncing) {
+            const room = client!.getRoom(roomId);
+            if (!room) {
+              // Member not (yet) joined to the room — provisioning still running.
+              setStatus("pending");
+              setPendingReason("room_not_provisioned");
+              return;
+            }
+            roomRef.current = room;
+            setStatus("ready");
+            refresh();
+          } else if (state === SyncState.Error) {
+            setStatus("error");
+            setError("Verbindung zum Chat-Server fehlgeschlagen.");
+          }
+        });
+
+        await client.startClient({ initialSyncLimit: PAGE_SIZE });
+      } catch (err) {
+        if (cancelled) return;
+        if (err instanceof ChatSetupPendingError) {
+          setStatus("pending");
+          setPendingReason(err.reason);
+          // Poll instead of erroring until provisioning caught up.
+          pollTimer = setTimeout(() => void connect(), POLL_INTERVAL_MS);
+          return;
+        }
+        setStatus("error");
+        setError((err as Error).message);
+      }
+    }
+
+    void connect();
+
+    return () => {
+      cancelled = true;
+      if (pollTimer) clearTimeout(pollTimer);
+      if (client) {
+        client.removeAllListeners();
+        client.stopClient();
+      }
+      clientRef.current = null;
+      roomRef.current = null;
+    };
+  }, [authToken, teamId, attempt, refresh]);
+
+  const sendMessage = useCallback(
+    async (body: string) => {
+      const client = clientRef.current;
+      const room = roomRef.current;
+      if (!client || !room || !body.trim()) return;
+      setSending(true);
+      setError(null);
+      try {
+        await client.sendTextMessage(room.roomId, body.trim());
+      } catch (err) {
+        // Expired token: one automatic retry with fresh credentials, then surface the error.
+        try {
+          const creds = await fetchChatCredentials(authToken!, { refresh: true });
+          client.setAccessToken(creds.accessToken);
+          await client.sendTextMessage(room.roomId, body.trim());
+        } catch {
+          setError((err as Error).message || "Nachricht konnte nicht gesendet werden.");
+          throw err;
+        }
+      } finally {
+        setSending(false);
+        refresh();
+      }
+    },
+    [authToken, refresh],
+  );
+
+  const loadMore = useCallback(async () => {
+    const client = clientRef.current;
+    const room = roomRef.current;
+    if (!client || !room || loadingMore) return;
+    setLoadingMore(true);
+    try {
+      await client.paginateEventTimeline(room.getLiveTimeline(), {
+        backwards: true,
+        limit: PAGE_SIZE,
+      });
+      refresh();
+    } catch (err) {
+      setError((err as Error).message);
+    } finally {
+      setLoadingMore(false);
+    }
+  }, [loadingMore, refresh]);
+
+  const markRead = useCallback(() => {
+    const client = clientRef.current;
+    const room = roomRef.current;
+    if (!client || !room) return;
+    const events = room.getLiveTimeline().getEvents();
+    const last: MatrixEvent | undefined = events[events.length - 1];
+    if (last) {
+      void client.sendReadReceipt(last).catch(() => undefined);
+      setUnreadCount(0);
+    }
+  }, []);
+
+  const retry = useCallback(() => setAttempt((n) => n + 1), []);
+
+  return {
+    status,
+    pendingReason,
+    error,
+    messages,
+    ownUserId,
+    unreadCount,
+    canLoadMore,
+    loadingMore,
+    sending,
+    sendMessage,
+    loadMore,
+    markRead,
+    retry,
+  };
+}
diff --git a/frontend/src/lib/matrix.ts b/frontend/src/lib/matrix.ts
new file mode 100644
index 0000000..4684c59
--- /dev/null
+++ b/frontend/src/lib/matrix.ts
@@ -0,0 +1,97 @@
+import { createClient, type MatrixClient } from "matrix-js-sdk";
+
+export interface ChatCredentials {
+  homeserverUrl: string;
+  userId: string;
+  accessToken: string;
+  expiresInMs: number;
+}
+
+export type ChatSetupReason = "matrix_not_provisioned" | "room_not_provisioned";
+
+export class ChatSetupPendingError extends Error {
+  constructor(public readonly reason: ChatSetupReason) {
+    super(reason);
+    this.name = "ChatSetupPendingError";
+  }
+}
+
+async function readError(res: Response): Promise<string> {
+  try {
+    const body = (await res.json()) as { error?: string };
+    return body.error ?? `Fehler (${res.status})`;
+  } catch {
+    return `Fehler (${res.status})`;
+  }
+}
+
+/** Asks the TalkTime backend for a short-lived Matrix access token for the logged-in user. */
+export async function fetchChatCredentials(
+  token: string,
+  options: { refresh?: boolean } = {},
+): Promise<ChatCredentials> {
+  const res = await fetch("/api/chat/token", {
+    method: "POST",
+    headers: {
+      Authorization: `Bearer ${token}`,
+      "Content-Type": "application/json",
+    },
+    // `refresh` forces the backend to drop its cached token instead of handing
+    // back the one Synapse just rejected.
+    body: JSON.stringify({ refresh: options.refresh === true }),
+  });
+  if (res.status === 409) {
+    throw new ChatSetupPendingError("matrix_not_provisioned");
+  }
+  if (!res.ok) {
+    throw new Error(await readError(res));
+  }
+  return (await res.json()) as ChatCredentials;
+}
+
+/** Resolves the Matrix room id of a team. */
+export async function fetchTeamRoomId(token: string, teamId: string): Promise<string> {
+  const res = await fetch(`/api/chat/room?teamId=${encodeURIComponent(teamId)}`, {
+    headers: { Authorization: `Bearer ${token}` },
+  });
+  if (res.status === 409) {
+    throw new ChatSetupPendingError("room_not_provisioned");
+  }
+  if (!res.ok) {
+    throw new Error(await readError(res));
+  }
+  const body = (await res.json()) as { matrixRoomId: string };
+  return body.matrixRoomId;
+}
+
+/**
+ * Creates a Matrix client that talks to Synapse directly.
+ * Chat traffic never passes through the TalkTime backend.
+ */
+export function createMatrixClient(creds: ChatCredentials): MatrixClient {
+  return createClient({
+    baseUrl: creds.homeserverUrl,
+    accessToken: creds.accessToken,
+    userId: creds.userId,
+  });
+}
+
+export function formatDayLabel(date: Date): string {
+  const today = new Date();
+  const yesterday = new Date();
+  yesterday.setDate(today.getDate() - 1);
+  const sameDay = (a: Date, b: Date) => a.toDateString() === b.toDateString();
+
+  if (sameDay(date, today)) return "Heute";
+  if (sameDay(date, yesterday)) return "Gestern";
+  return date.toLocaleDateString("de-DE", {
+    weekday: "long",
+    day: "2-digit",
+    month: "2-digit",
+    year: "numeric",
+  });
+}
+
+export function formatTime(date: Date): string {
+  return date.toLocaleTimeString("de-DE", { hour: "2-digit", minute: "2-digit" });
+}
diff --git a/frontend/src/pages/TeamsPage.tsx b/frontend/src/pages/TeamsPage.tsx
index 7bff973..c1e1e43 100644
--- a/frontend/src/pages/TeamsPage.tsx
+++ b/frontend/src/pages/TeamsPage.tsx
@@ -1,4 +1,4 @@
-import { useState } from "react";
+import { Suspense, lazy, useState } from "react";
 import { Link } from "react-router-dom";
 import { Button } from "@/components/ui/button";
 import { Input } from "@/components/ui/input";
@@ -13,6 +13,10 @@ import {
 import { useAuth } from "@/lib/auth-context";
 import { useOrganization } from "@/lib/organization-context";
 import { useApiList } from "@/hooks/use-api-list";
+// Lazy: pulls in matrix-js-sdk only once a team is actually opened.
+const TeamDetail = lazy(() =>
+  import("@/components/chat/TeamDetail").then((m) => ({ default: m.TeamDetail })),
+);
 
 interface Department {
   id: string;
@@ -36,6 +40,7 @@ export function TeamsPage() {
   const [departmentId, setDepartmentId] = useState<string>("");
   const [seasonId, setSeasonId] = useState<string>("");
   const [error, setError] = useState<string | null>(null);
+  const [selectedTeamId, setSelectedTeamId] = useState<string | null>(null);
 
   const orgQuery = activeOrganizationId ? `organizationId=${activeOrganizationId}` : null;
   const { data: departments } = useApiList<Department>(
@@ -79,6 +84,8 @@ export function TeamsPage() {
     }
   }
 
+  const selectedTeam = teams.find((t) => t.id === selectedTeamId) ?? null;
+
   const departmentName = (id: string) => departments.find((d) => d.id === id)?.name ?? "?";
   const seasonName = (id: string) => seasons.find((s) => s.id === id)?.name ?? "?";
 
@@ -140,7 +147,22 @@ export function TeamsPage() {
           <p className="text-sm text-muted-foreground">Keine Teams</p>
         ) : (
           teams.map((team) => (
-            <Card key={team.id}>
+            <Card
+              key={team.id}
+              role="button"
+              tabIndex={0}
+              aria-pressed={selectedTeamId === team.id}
+              onClick={() => setSelectedTeamId(team.id)}
+              onKeyDown={(e) => {
+                if (e.key === "Enter" || e.key === " ") {
+                  e.preventDefault();
+                  setSelectedTeamId(team.id);
+                }
+              }}
+              className={`cursor-pointer transition-colors hover:bg-accent ${
+                selectedTeamId === team.id ? "ring-2 ring-ring" : ""
+              }`}
+            >
               <CardContent className="pt-6 space-y-1">
                 <p className="font-medium">{team.name}</p>
                 <p className="text-xs text-muted-foreground">
@@ -151,6 +173,18 @@ export function TeamsPage() {
           ))
         )}
       </div>
+
+      {selectedTeam && (
+        <Suspense fallback={<p className="text-sm text-muted-foreground">Lädt…</p>}>
+          <TeamDetail
+          teamId={selectedTeam.id}
+          teamName={selectedTeam.name}
+          departmentName={departmentName(selectedTeam.departmentId)}
+          seasonName={seasonName(selectedTeam.seasonId)}
+            onClose={() => setSelectedTeamId(null)}
+          />
+        </Suspense>
+      )}
     </main>
   );
 }