diff --git a/src/lib/auth.ts b/src/lib/auth.ts
index 34a42e2..ebd2960 100644
--- a/src/lib/auth.ts
+++ b/src/lib/auth.ts
@@ -5,7 +5,11 @@ import { Strategy as LocalStrategy } from "passport-local";
 import type { Config } from "./config.js";
 import type { Db } from "./db.js";
 import { hashPassword, verifyPassword } from "./passwords.js";
-import { PgSessionStore } from "./session-store.js";
+import {
+  MemorySessionStore,
+  PgSessionStore,
+  type RevocableStore,
+} from "./session-store.js";
 import type { User, UserStore } from "./user-store.js";
 
 declare global {
@@ -28,6 +32,8 @@ export interface Auth {
   /** session + passport.initialize + passport.session, in order. */
   middleware: RequestHandler[];
   authenticate: RequestHandler;
+  /** Drop every session of `userId` except the one with id `exceptSid`. */
+  revokeOtherSessions(userId: string, exceptSid: string): Promise<void>;
 }
 
 export function createAuth(opts: {
@@ -65,6 +71,9 @@ export function createAuth(opts: {
     }
   });
 
+  const store: RevocableStore = db
+    ? new PgSessionStore(db, SESSION_TTL_MS)
+    : new MemorySessionStore();
   const secret =
     config.SESSION_SECRET ?? "dev-only-secret-not-for-production-use";
   return {
@@ -74,7 +83,7 @@ export function createAuth(opts: {
         secret,
         resave: false,
         saveUninitialized: false,
-        store: db ? new PgSessionStore(db, SESSION_TTL_MS) : undefined,
+        store,
         cookie: {
           httpOnly: true,
           sameSite: "lax",
@@ -85,6 +94,8 @@ export function createAuth(opts: {
       passport.initialize(),
       passport.session(),
     ],
+    revokeOtherSessions: (userId, exceptSid) =>
+      store.destroyForUser(userId, exceptSid),
     authenticate: (req, res, next) =>
       passport.authenticate("local", (err: unknown, user?: User | false) => {
         if (err) return next(err);
diff --git a/src/lib/session-store.ts b/src/lib/session-store.ts
index 2a195c3..103999a 100644
--- a/src/lib/session-store.ts
+++ b/src/lib/session-store.ts
@@ -61,6 +61,14 @@ export class PgSessionStore extends session.Store {
       );
   }
 
+  /** Sign a user out everywhere except `exceptSid` (e.g. after a password change). */
+  async destroyForUser(userId: string, exceptSid: string): Promise<void> {
+    await this.db.query(
+      "DELETE FROM sessions WHERE sess->'passport'->>'user' = $1 AND sid <> $2",
+      [userId, exceptSid],
+    );
+  }
+
   destroy(sid: string, cb?: (err?: unknown) => void) {
     this.db.query("DELETE FROM sessions WHERE sid = $1", [sid]).then(
       () => cb?.(),
@@ -68,3 +76,24 @@ export class PgSessionStore extends session.Store {
     );
   }
 }
+
+/** In-memory sessions (tests / no database), with the same revoke hook. */
+export class MemorySessionStore extends session.MemoryStore {
+  async destroyForUser(userId: string, exceptSid: string): Promise<void> {
+    const all = await new Promise<Record<string, session.SessionData>>(
+      (resolve, reject) =>
+        this.all((err, sessions) =>
+          err ? reject(err) : resolve(sessions ?? {}),
+        ),
+    );
+    for (const [sid, sess] of Object.entries(all)) {
+      const passport = (sess as { passport?: { user?: string } }).passport;
+      if (sid !== exceptSid && passport?.user === userId) {
+        await new Promise<void>((r) => this.destroy(sid, () => r()));
+      }
+    }
+  }
+}
+
+export type RevocableStore = (PgSessionStore | MemorySessionStore) &
+  session.Store;
diff --git a/src/lib/user-store.ts b/src/lib/user-store.ts
index 14245d2..76716b7 100644
--- a/src/lib/user-store.ts
+++ b/src/lib/user-store.ts
@@ -16,6 +16,8 @@ export interface UserStore {
   create(username: string, passwordHash: string): Promise<User>;
   findByUsername(username: string): Promise<UserWithHash | null>;
   findById(id: string): Promise<User | null>;
+  findWithHashById(id: string): Promise<UserWithHash | null>;
+  updatePassword(id: string, passwordHash: string): Promise<void>;
 }
 
 export function memoryUserStore(): UserStore {
@@ -38,6 +40,13 @@ export function memoryUserStore(): UserStore {
       const u = rows.get(id);
       return u ? { id: u.id, username: u.username } : null;
     },
+    async findWithHashById(id) {
+      return rows.get(id) ?? null;
+    },
+    async updatePassword(id, passwordHash) {
+      const u = rows.get(id);
+      if (u) u.passwordHash = passwordHash;
+    },
   };
 }
 
@@ -85,5 +94,25 @@ export function pgUserStore(db: Db): UserStore {
       const row = r.rows[0];
       return row ? { id: row.id, username: row.username } : null;
     },
+    async findWithHashById(id) {
+      const r = await db.query<Row>(
+        "SELECT id, username, password_hash FROM users WHERE id = $1",
+        [id],
+      );
+      const row = r.rows[0];
+      return row
+        ? {
+            id: row.id,
+            username: row.username,
+            passwordHash: row.password_hash,
+          }
+        : null;
+    },
+    async updatePassword(id, passwordHash) {
+      await db.query("UPDATE users SET password_hash = $2 WHERE id = $1", [
+        id,
+        passwordHash,
+      ]);
+    },
   };
 }
diff --git a/src/routes/auth.test.ts b/src/routes/auth.test.ts
index 9d4b091..c311682 100644
--- a/src/routes/auth.test.ts
+++ b/src/routes/auth.test.ts
@@ -89,6 +89,18 @@ describe("auth API", () => {
     expect((await agent.get("/auth/me")).body).toEqual({ user: null });
   });
 
+  it("lets the same browser log back in after logging out", async () => {
+    const agent = request.agent(mk());
+    await agent.post("/auth/register").send(creds);
+    expect((await agent.post("/auth/logout")).status).toBe(204);
+    const again = await agent.post("/auth/login").send(creds);
+    expect(again.status).toBe(200);
+    expect((await agent.get("/auth/me")).body.user.username).toBe("alice");
+    // and a second logout/login cycle works too
+    await agent.post("/auth/logout");
+    expect((await agent.post("/auth/login").send(creds)).status).toBe(200);
+  });
+
   it("rotates the session id on login", async () => {
     const app = mk();
     await request(app).post("/auth/register").send(creds);
@@ -136,6 +148,101 @@ describe("auth API", () => {
   });
 });
 
+describe("change password", () => {
+  const next = "a brand new passphrase";
+
+  it("requires login", async () => {
+    const r = await request(mk())
+      .post("/auth/password")
+      .send({ currentPassword: creds.password, newPassword: next });
+    expect(r.status).toBe(401);
+  });
+
+  it("changes the password: old stops working, new works, this session stays", async () => {
+    const app = mk();
+    const agent = request.agent(app);
+    await agent.post("/auth/register").send(creds);
+    const r = await agent
+      .post("/auth/password")
+      .send({ currentPassword: creds.password, newPassword: next });
+    expect(r.status).toBe(204);
+    expect((await agent.get("/auth/me")).body.user.username).toBe("alice");
+    await agent.post("/auth/logout");
+    expect((await request(app).post("/auth/login").send(creds)).status).toBe(
+      401,
+    );
+    expect(
+      (
+        await request(app)
+          .post("/auth/login")
+          .send({ username: creds.username, password: next })
+      ).status,
+    ).toBe(200);
+  });
+
+  it("signs out the user's other sessions but not other users'", async () => {
+    const app = mk();
+    const phone = request.agent(app);
+    const laptop = request.agent(app);
+    const bob = request.agent(app);
+    await phone.post("/auth/register").send(creds);
+    await laptop.post("/auth/login").send(creds);
+    await bob
+      .post("/auth/register")
+      .send({ username: "bob", password: "bobs passphrase" });
+    expect((await phone.get("/auth/me")).body.user).not.toBeNull();
+
+    await laptop
+      .post("/auth/password")
+      .send({ currentPassword: creds.password, newPassword: next });
+
+    expect((await laptop.get("/auth/me")).body.user.username).toBe("alice");
+    expect((await phone.get("/auth/me")).body.user).toBeNull();
+    expect((await bob.get("/auth/me")).body.user.username).toBe("bob");
+  });
+
+  it("rejects a wrong current password (403), a weak or unchanged new one (400)", async () => {
+    const agent = request.agent(mk());
+    await agent.post("/auth/register").send(creds);
+    const post = (body: object) => agent.post("/auth/password").send(body);
+    expect(
+      (await post({ currentPassword: "wrong wrong", newPassword: next }))
+        .status,
+    ).toBe(403);
+    expect(
+      (await post({ currentPassword: creds.password, newPassword: "short" }))
+        .status,
+    ).toBe(400);
+    expect(
+      (
+        await post({
+          currentPassword: creds.password,
+          newPassword: creds.password,
+        })
+      ).status,
+    ).toBe(400);
+    expect((await post({})).status).toBe(400);
+    // nothing changed
+    await agent.post("/auth/logout");
+    expect((await agent.post("/auth/login").send(creds)).status).toBe(200);
+  });
+
+  it("locks out guessing of the current password", async () => {
+    const agent = request.agent(mk());
+    await agent.post("/auth/register").send(creds);
+    const guess = () =>
+      agent
+        .post("/auth/password")
+        .send({ currentPassword: "wrong wrong", newPassword: next });
+    for (let i = 0; i < 10; i++) expect((await guess()).status).toBe(403);
+    expect((await guess()).status).toBe(429);
+    const right = await agent
+      .post("/auth/password")
+      .send({ currentPassword: creds.password, newPassword: next });
+    expect(right.status).toBe(429);
+  });
+});
+
 describe("config", () => {
   it("requires SESSION_SECRET once DATABASE_URL is set", () => {
     const base = { SERVICE_NAME: "t", DATABASE_URL: "postgres://u:p@h/db" };
diff --git a/src/routes/auth.ts b/src/routes/auth.ts
index 01d5148..79c15e2 100644
--- a/src/routes/auth.ts
+++ b/src/routes/auth.ts
@@ -1,7 +1,7 @@
-import express, { Router, type RequestHandler } from "express";
+import express, { Router, type Request, type RequestHandler } from "express";
 import { z } from "zod";
-import type { Auth } from "../lib/auth.js";
-import { hashPassword } from "../lib/passwords.js";
+import { requireAuth, type Auth } from "../lib/auth.js";
+import { hashPassword, verifyPassword } from "../lib/passwords.js";
 import { UsernameTakenError, type UserStore } from "../lib/user-store.js";
 
 const credentials = z.object({
@@ -11,13 +11,16 @@ const credentials = z.object({
   password: z.string().min(8, "at least 8 characters").max(128),
 });
 
-/** Failed logins per (ip, username): 10 per 15 min, per pod. Cheap brute-force brake. */
-function loginLimiter(): RequestHandler {
+/** Failures (responses with `failStatus`) per key: 10 per 15 min, per pod. Cheap brute-force brake. */
+function failureLimiter(
+  keyOf: (req: Request) => string,
+  failStatus: number,
+): RequestHandler {
   const WINDOW = 15 * 60_000;
   const MAX = 10;
   const fails = new Map<string, { n: number; reset: number }>();
   return (req, res, next) => {
-    const key = `${req.ip}|${String(req.body?.username ?? "").toLowerCase()}`;
+    const key = keyOf(req);
     const now = Date.now();
     const e = fails.get(key);
     if (e && e.reset > now && e.n >= MAX) {
@@ -25,7 +28,7 @@ function loginLimiter(): RequestHandler {
       return;
     }
     res.on("finish", () => {
-      if (res.statusCode === 401) {
+      if (res.statusCode === failStatus) {
         const cur = fails.get(key);
         if (!cur || cur.reset <= Date.now()) {
           fails.set(key, { n: 1, reset: Date.now() + WINDOW });
@@ -83,10 +86,68 @@ export function authRouter(deps: {
     }
   });
 
-  router.post("/auth/login", json, loginLimiter(), (req, res, next) => {
+  const loginLimiter = failureLimiter(
+    (req) => `${req.ip}|${String(req.body?.username ?? "").toLowerCase()}`,
+    401,
+  );
+  const passwordLimiter = failureLimiter(
+    (req) => `pw|${req.user?.id ?? req.ip}`,
+    403,
+  );
+
+  router.post("/auth/login", json, loginLimiter, (req, res, next) => {
     deps.auth.authenticate(req, res, next);
   });
 
+  const passwordChange = z.object({
+    currentPassword: z.string().min(1).max(128),
+    newPassword: credentials.shape.password,
+  });
+
+  router.post(
+    "/auth/password",
+    requireAuth,
+    json,
+    passwordLimiter,
+    async (req, res, next) => {
+      try {
+        const parsed = passwordChange.safeParse(req.body);
+        if (!parsed.success) {
+          return void res.status(400).json({
+            error: parsed.error.issues
+              .map((i) => `${i.path.join(".")}: ${i.message}`)
+              .join("; "),
+          });
+        }
+        const { currentPassword, newPassword } = parsed.data;
+        const user = await deps.users.findWithHashById(req.user!.id);
+        if (
+          !user ||
+          !(await verifyPassword(currentPassword, user.passwordHash))
+        ) {
+          return void res
+            .status(403)
+            .json({ error: "current password is incorrect" });
+        }
+        if (currentPassword === newPassword) {
+          return void res
+            .status(400)
+            .json({ error: "new password must differ from the current one" });
+        }
+        await deps.users.updatePassword(
+          user.id,
+          await hashPassword(newPassword),
+        );
+        // Other devices (or a thief holding an old session) are signed out;
+        // this one stays signed in.
+        await deps.auth.revokeOtherSessions(user.id, req.sessionID);
+        res.status(204).end();
+      } catch (e) {
+        next(e);
+      }
+    },
+  );
+
   router.post("/auth/logout", (req, res, next) => {
     req.logout((err) => {
       if (err) return next(err);