diff --git a/portal/app/TwoFactorField.tsx b/portal/app/TwoFactorField.tsx
new file mode 100644
index 0000000..701db88
--- /dev/null
+++ b/portal/app/TwoFactorField.tsx
@@ -0,0 +1,56 @@
+"use client";
+
+import type { TwoFactorProof } from "../lib/api";
+
+export interface TwoFactorEntry {
+  recovery: boolean;
+  value: string;
+}
+
+export const EMPTY_TWO_FACTOR_ENTRY: TwoFactorEntry = { recovery: false, value: "" };
+
+export function entryProof(entry: TwoFactorEntry): TwoFactorProof | null {
+  const value = entry.value.trim();
+  if (!value) return null;
+  return entry.recovery ? { recoveryCode: value } : { code: value };
+}
+
+const ALL_METHODS = ["totp", "recovery"];
+
+export function TwoFactorField({ entry, onChange, methods = ALL_METHODS, autoFocus }: {
+  entry: TwoFactorEntry;
+  onChange: (entry: TwoFactorEntry) => void;
+  /** What the server will accept; one that lost its seed key offers recovery codes only. */
+  methods?: string[];
+  autoFocus?: boolean;
+}) {
+  const recovery = entry.recovery || !methods.includes("totp");
+  const canSwitch = methods.includes("totp") && methods.includes("recovery");
+  return (
+    <div className="flex flex-col gap-1.5">
+      <input
+        key={recovery ? "recovery" : "totp"}
+        type="text"
+        value={entry.value}
+        onChange={(e) => onChange({ recovery, value: e.target.value })}
+        autoFocus={autoFocus}
+        autoComplete={recovery ? "off" : "one-time-code"}
+        inputMode={recovery ? "text" : "numeric"}
+        autoCapitalize="none"
+        spellCheck={false}
+        maxLength={recovery ? 32 : 8}
+        placeholder={recovery ? "Recovery code" : "6-digit code"}
+        className="w-full px-3.5 py-2.5 rounded-xl bg-[#0a0a0f] border border-[#1e1e2e] text-sm font-mono tracking-wider text-white placeholder:text-zinc-600 focus:outline-none focus:border-cyan-500/50 focus:ring-1 focus:ring-cyan-500/20 transition-colors"
+      />
+      {canSwitch && (
+        <button
+          type="button"
+          onClick={() => onChange({ recovery: !recovery, value: "" })}
+          className="self-start text-xs text-cyan-400 hover:text-cyan-300 transition-colors"
+        >
+          {recovery ? "Use an authenticator code" : "Use a recovery code"}
+        </button>
+      )}
+    </div>
+  );
+}
diff --git a/portal/app/account/ChangePasswordModal.tsx b/portal/app/account/ChangePasswordModal.tsx
index b1f764e..ed5f134 100644
--- a/portal/app/account/ChangePasswordModal.tsx
+++ b/portal/app/account/ChangePasswordModal.tsx
@@ -2,7 +2,8 @@
 
 import { useState, type FormEvent } from "react";
 import { deriveAuthKey, deriveKek, wrapUserSecrets, base64ToBytes, bytesToBase64, unwrapUserSecretsDek, unwrapUserSecretsX25519 } from "../../lib/crypto";
-import { getMe, updatePassword, ApiError } from "../../lib/api";
+import { getMe, updatePassword, twoFactorMessage, ApiError } from "../../lib/api";
+import { EMPTY_TWO_FACTOR_ENTRY, TwoFactorField, entryProof, type TwoFactorEntry } from "../TwoFactorField";
 
 interface Props {
   accountId: string;
@@ -18,6 +19,8 @@ export default function ChangePasswordModal({ accountId, token, onClose, onSucce
   const [loading, setLoading] = useState(false);
   const [error, setError] = useState("");
   const [done, setDone] = useState(false);
+  const [needsCode, setNeedsCode] = useState(false);
+  const [twoFactor, setTwoFactor] = useState<TwoFactorEntry>(EMPTY_TWO_FACTOR_ENTRY);
 
   async function handleSubmit(e: FormEvent) {
     e.preventDefault();
@@ -35,6 +38,13 @@ export default function ChangePasswordModal({ accountId, token, onClose, onSucce
         setLoading(false);
         return;
       }
+      const proof = entryProof(twoFactor);
+      if (me.two_factor_enabled && !proof) {
+        setNeedsCode(true);
+        setError("Enter a code from your authenticator app to confirm the change.");
+        setLoading(false);
+        return;
+      }
 
       const oldKek = await deriveKek(currentPassword, accountId);
       const wrappedBytes = base64ToBytes(me.wrapped_user_secrets);
@@ -52,11 +62,16 @@ export default function ChangePasswordModal({ accountId, token, onClose, onSucce
       const newWrappedBytes = await wrapUserSecrets(newKek, dek, x25519Private);
       const newWrappedUserSecrets = bytesToBase64(newWrappedBytes);
 
-      const auth = await updatePassword(oldAuthKey, newAuthKey, newWrappedUserSecrets, token);
+      const auth = await updatePassword(oldAuthKey, newAuthKey, newWrappedUserSecrets, token, proof);
       setDone(true);
       onSuccess(auth.jwt_token, auth.refresh_token);
     } catch (err) {
-      if (err instanceof ApiError && err.status === 401) {
+      const refusal = twoFactorMessage(err);
+      if (refusal) {
+        setNeedsCode(true);
+        setError(refusal);
+        setTwoFactor((entry) => ({ ...entry, value: "" }));
+      } else if (err instanceof ApiError && err.status === 401) {
         setError("Current password is incorrect.");
       } else {
         setError(err instanceof Error ? err.message : "Password change failed.");
@@ -84,6 +99,7 @@ export default function ChangePasswordModal({ accountId, token, onClose, onSucce
               className="w-full px-3.5 py-2.5 rounded-xl bg-[#0a0a0f] border border-[#1e1e2e] text-sm text-white placeholder:text-zinc-600 focus:outline-none focus:border-cyan-500/50 focus:ring-1 focus:ring-cyan-500/20 transition-colors" />
             <input type="password" placeholder="Confirm new password" value={confirmPassword} onChange={(e) => setConfirmPassword(e.target.value)}
               className="w-full px-3.5 py-2.5 rounded-xl bg-[#0a0a0f] border border-[#1e1e2e] text-sm text-white placeholder:text-zinc-600 focus:outline-none focus:border-cyan-500/50 focus:ring-1 focus:ring-cyan-500/20 transition-colors" />
+            {needsCode && <TwoFactorField entry={twoFactor} onChange={setTwoFactor} autoFocus />}
             {error && <p className="text-sm text-red-400">{error}</p>}
             <div className="flex gap-2 mt-1">
               <button type="button" onClick={onClose} className="flex-1 py-2.5 rounded-xl border border-[#1e1e2e] hover:border-zinc-600 text-zinc-400 hover:text-white text-sm transition-colors">Cancel</button>
diff --git a/portal/app/signin/page.tsx b/portal/app/signin/page.tsx
index c248f64..0e3deb0 100644
--- a/portal/app/signin/page.tsx
+++ b/portal/app/signin/page.tsx
@@ -4,7 +4,11 @@ import { useState, useEffect, useRef } from "react";
 import { useRouter } from "next/navigation";
 import Image from "next/image";
 import { deriveAuthKey } from "../../lib/crypto";
-import { getChallenge, login } from "../../lib/api";
+import {
+  ApiError, getChallenge, login, loginSecondFactor, twoFactorMessage,
+  type AuthResponse, type TwoFactorChallenge,
+} from "../../lib/api";
+import { EMPTY_TWO_FACTOR_ENTRY, TwoFactorField, entryProof, type TwoFactorEntry } from "../TwoFactorField";
 
 export default function SigninPage() {
   const router = useRouter();
@@ -12,32 +16,64 @@ export default function SigninPage() {
   const [password, setPassword] = useState("");
   const [error, setError] = useState("");
   const [loading, setLoading] = useState(false);
+  const [challenge, setChallenge] = useState<TwoFactorChallenge | null>(null);
+  const [twoFactor, setTwoFactor] = useState<TwoFactorEntry>(EMPTY_TWO_FACTOR_ENTRY);
   const emailRef = useRef<HTMLInputElement>(null);
 
   useEffect(() => { emailRef.current?.focus(); }, []);
 
-  async function handleSubmit(e: React.FormEvent) {
-    e.preventDefault();
+  function enter(auth: AuthResponse) {
+    sessionStorage.setItem("access_token", auth.jwt_token);
+    sessionStorage.setItem("refresh_token", auth.refresh_token);
+    sessionStorage.setItem("tier", auth.tier);
+    if (auth.trial_ends_at) sessionStorage.setItem("trial_ends_at", String(auth.trial_ends_at));
+
+    router.push("/account");
+  }
+
+  function backToPassword(message = "") {
+    setChallenge(null);
+    setTwoFactor(EMPTY_TWO_FACTOR_ENTRY);
+    setError(message);
+  }
+
+  async function attempt(work: () => Promise<void>) {
     setError("");
     setLoading(true);
     try {
-      const { account_id } = await getChallenge(email);
-      const authKey = await deriveAuthKey(password, account_id);
-      const auth = await login(authKey, account_id);
-
-      sessionStorage.setItem("access_token", auth.jwt_token);
-      sessionStorage.setItem("refresh_token", auth.refresh_token);
-      sessionStorage.setItem("tier", auth.tier);
-      if (auth.trial_ends_at) sessionStorage.setItem("trial_ends_at", String(auth.trial_ends_at));
-
-      router.push("/account");
+      await work();
     } catch (err) {
-      setError(err instanceof Error ? err.message : "Sign in failed.");
+      const refusal = twoFactorMessage(err);
+      // A bare 401 at the code step is the five-minute pending sign-in running out.
+      if (!refusal && challenge && err instanceof ApiError && err.status === 401) {
+        backToPassword("This sign-in took too long. Start again.");
+      } else {
+        setError(refusal ?? (err instanceof Error ? err.message : "Sign in failed."));
+        setTwoFactor((entry) => ({ ...entry, value: "" }));
+      }
     } finally {
       setLoading(false);
     }
   }
 
+  function handleSubmit(e: React.FormEvent) {
+    e.preventDefault();
+    return attempt(async () => {
+      const { account_id } = await getChallenge(email);
+      const authKey = await deriveAuthKey(password, account_id);
+      const result = await login(authKey, account_id);
+      if ("challenge" in result) setChallenge(result.challenge);
+      else enter(result.session);
+    });
+  }
+
+  function handleCode(e: React.FormEvent) {
+    e.preventDefault();
+    const proof = entryProof(twoFactor);
+    if (!challenge || !proof) return;
+    return attempt(async () => enter(await loginSecondFactor(challenge.pending_token, proof)));
+  }
+
   return (
     <main className="relative min-h-screen flex items-center justify-center px-4 overflow-hidden">
       {/* Ambient glow */}
@@ -56,11 +92,37 @@ export default function SigninPage() {
 
         {/* Card */}
         <div className="rounded-2xl border border-[#1e1e2e] bg-[#111118] p-8 shadow-[0_0_60px_rgba(6,182,212,0.04)]">
-          <h1 className="text-xl font-bold text-white mb-1">Welcome back</h1>
+          <h1 className="text-xl font-bold text-white mb-1">{challenge ? "Two-factor authentication" : "Welcome back"}</h1>
           <p className="text-sm text-zinc-500 mb-6">
-            Your password never leaves your device.
+            {challenge
+              ? "Enter the code from your authenticator app, or one of your recovery codes."
+              : "Your password never leaves your device."}
           </p>
 
+          {challenge ? (
+            <form onSubmit={handleCode} className="flex flex-col gap-4">
+              <TwoFactorField entry={twoFactor} onChange={setTwoFactor} methods={challenge.methods} autoFocus />
+
+              {error && (
+                <p className="text-sm text-red-400 -mt-1">{error}</p>
+              )}
+
+              <button
+                type="submit"
+                disabled={loading || !twoFactor.value.trim()}
+                className="mt-1 w-full py-3 rounded-xl bg-cyan-500 hover:bg-cyan-400 disabled:bg-[#1e1e2e] disabled:text-zinc-500 disabled:cursor-default text-black font-semibold text-sm transition-all duration-200 hover:shadow-[0_0_24px_rgba(6,182,212,0.4)]"
+              >
+                {loading ? "Verifying…" : "Verify"}
+              </button>
+              <button
+                type="button"
+                onClick={() => backToPassword()}
+                className="text-sm text-zinc-500 hover:text-zinc-300 transition-colors"
+              >
+                Back
+              </button>
+            </form>
+          ) : (
           <form onSubmit={handleSubmit} className="flex flex-col gap-4">
             <Field label="Email" type="email" value={email} onChange={setEmail} inputRef={emailRef} required />
             <Field label="Master password" type="password" value={password} onChange={setPassword} required />
@@ -77,6 +139,7 @@ export default function SigninPage() {
               {loading ? "Signing in…" : "Sign in"}
             </button>
           </form>
+          )}
         </div>
 
         <p className="mt-5 text-center text-sm text-zinc-500">
diff --git a/portal/lib/api.ts b/portal/lib/api.ts
index f923c53..41df15f 100644
--- a/portal/lib/api.ts
+++ b/portal/lib/api.ts
@@ -8,6 +8,16 @@ export class ApiError extends Error {
     super(message);
     this.name = "ApiError";
   }
+
+  /** The `{"error": CODE, ...}` body the server sends with a refusal it wants told apart. */
+  get body(): Record<string, unknown> | null {
+    try {
+      const parsed: unknown = JSON.parse(this.message);
+      return parsed && typeof parsed === "object" ? (parsed as Record<string, unknown>) : null;
+    } catch {
+      return null;
+    }
+  }
 }
 
 async function request<T>(
@@ -55,8 +65,37 @@ export interface MeResponse {
   handle: string;
   handle_is_custom: boolean;
   handle_managed?: boolean;
+  two_factor_enabled?: boolean;
+}
+
+export type TwoFactorProof = { code: string } | { recoveryCode: string };
+
+function proofBody(proof?: TwoFactorProof | null): { code?: string; recovery_code?: string } {
+  if (!proof) return {};
+  return "code" in proof ? { code: proof.code.trim() } : { recovery_code: proof.recoveryCode.trim() };
+}
+
+const TWO_FACTOR_MESSAGES: Record<string, string> = {
+  TWO_FACTOR_REQUIRED: "Enter the code from your authenticator app.",
+  TWO_FACTOR_INVALID: "Wrong code. Try the next one your app shows.",
+  TWO_FACTOR_LOCKED: "Too many wrong codes. Try again in 15 minutes.",
+  TWO_FACTOR_UNAVAILABLE: "This server cannot check authenticator codes right now. Use a recovery code.",
+};
+
+/** What to tell the user when a refusal is about the second factor, else null. */
+export function twoFactorMessage(err: unknown): string | null {
+  const code = err instanceof ApiError ? err.body?.error : null;
+  return typeof code === "string" ? (TWO_FACTOR_MESSAGES[code] ?? null) : null;
+}
+
+export interface TwoFactorChallenge {
+  pending_token: string;
+  /** "totp" and/or "recovery". */
+  methods: string[];
 }
 
+export type LoginResult = { session: AuthResponse } | { challenge: TwoFactorChallenge };
+
 export interface CheckoutResponse {
   checkout_url: string;
 }
@@ -115,6 +154,7 @@ export function updatePassword(
   newAuthKey: string,
   newWrappedUserSecrets: string,
   token: string,
+  proof?: TwoFactorProof | null,
 ): Promise<AuthResponse> {
   return request<AuthResponse>(
     "/v1/auth/password",
@@ -124,16 +164,37 @@ export function updatePassword(
         old_auth_key: oldAuthKey,
         new_auth_key: newAuthKey,
         new_wrapped_user_secrets: newWrappedUserSecrets,
+        ...proofBody(proof),
       }),
     },
     token,
   );
 }
 
-export function login(authKey: string, accountId: string): Promise<AuthResponse> {
-  return request<AuthResponse>("/v1/auth/login", {
+/** A session, or the challenge an account with two-factor sign-in answers the password with. */
+export async function login(authKey: string, accountId: string): Promise<LoginResult> {
+  try {
+    const session = await request<AuthResponse>("/v1/auth/login", {
+      method: "POST",
+      body: JSON.stringify({ auth_key: authKey, account_id: accountId }),
+    });
+    return { session };
+  } catch (err) {
+    const body = err instanceof ApiError && err.status === 428 ? err.body : null;
+    if (body?.error !== "TWO_FACTOR_REQUIRED" || typeof body.pending_token !== "string") throw err;
+    return {
+      challenge: {
+        pending_token: body.pending_token,
+        methods: Array.isArray(body.methods) ? body.methods.filter((m): m is string => typeof m === "string") : ["totp"],
+      },
+    };
+  }
+}
+
+export function loginSecondFactor(pendingToken: string, proof: TwoFactorProof): Promise<AuthResponse> {
+  return request<AuthResponse>("/v1/auth/login/2fa", {
     method: "POST",
-    body: JSON.stringify({ auth_key: authKey, account_id: accountId }),
+    body: JSON.stringify({ pending_token: pendingToken, ...proofBody(proof) }),
   });
 }