diff --git a/iosApp/honeyDue.xcodeproj/project.pbxproj b/iosApp/honeyDue.xcodeproj/project.pbxproj
index 28607f8..4fc98b0 100644
--- a/iosApp/honeyDue.xcodeproj/project.pbxproj
+++ b/iosApp/honeyDue.xcodeproj/project.pbxproj
@@ -758,6 +758,7 @@
 				INFOPLIST_KEY_ITSAppUsesNonExemptEncryption = NO;
 				INFOPLIST_KEY_LSSupportsOpeningDocumentsInPlace = NO;
 				INFOPLIST_KEY_NSCameraUsageDescription = "honeyDue needs camera access to take photos of tasks, documents, and receipts.";
+				INFOPLIST_KEY_NSFaceIDUsageDescription = "honeyDue uses Face ID to unlock the app.";
 				INFOPLIST_KEY_NSPhotoLibraryAddUsageDescription = "honeyDue needs permission to save photos to your library.";
 				INFOPLIST_KEY_NSPhotoLibraryUsageDescription = "honeyDue needs photo library access to attach photos to tasks and documents.";
 				INFOPLIST_KEY_UIApplicationSceneManifest_Generation = YES;
@@ -1246,6 +1247,7 @@
 				INFOPLIST_KEY_ITSAppUsesNonExemptEncryption = NO;
 				INFOPLIST_KEY_LSSupportsOpeningDocumentsInPlace = NO;
 				INFOPLIST_KEY_NSCameraUsageDescription = "honeyDue needs camera access to take photos of tasks, documents, and receipts.";
+				INFOPLIST_KEY_NSFaceIDUsageDescription = "honeyDue uses Face ID to unlock the app.";
 				INFOPLIST_KEY_NSPhotoLibraryAddUsageDescription = "honeyDue needs permission to save photos to your library.";
 				INFOPLIST_KEY_NSPhotoLibraryUsageDescription = "honeyDue needs photo library access to attach photos to tasks and documents.";
 				INFOPLIST_KEY_UIApplicationSceneManifest_Generation = YES;
diff --git a/iosApp/iosApp/AppLock/AppLockManager.swift b/iosApp/iosApp/AppLock/AppLockManager.swift
new file mode 100644
index 0000000..43c551d
--- /dev/null
+++ b/iosApp/iosApp/AppLock/AppLockManager.swift
@@ -0,0 +1,125 @@
+import Foundation
+import LocalAuthentication
+import CryptoKit
+
+/// App-lock: gates an already-authenticated session behind Face ID / Touch ID
+/// with a numeric-PIN fallback. Sits ABOVE auth — RootView overlays the lock
+/// screen when an authenticated user has it enabled and it's armed. All state
+/// lives in the Keychain (via KeychainHelper). Fully disabled under UI tests so
+/// the XCUITest suite is never gated by a lock screen.
+@MainActor
+final class AppLockManager: ObservableObject {
+    static let shared = AppLockManager()
+
+    static let pinLength = 6
+
+    private enum Key {
+        static let enabled = "app_lock_enabled"
+        static let pinHash = "app_lock_pin_hash"
+        static let biometric = "app_lock_biometric_enabled"
+    }
+
+    /// True when the lock screen should cover the app.
+    @Published private(set) var isLocked: Bool = false
+
+    private let keychain = KeychainHelper.shared
+
+    private init() {
+        // Locked on cold launch if the user enabled it.
+        isLocked = isEnabled
+    }
+
+    /// Whether app-lock is on. Always false under UI tests.
+    var isEnabled: Bool {
+        if UITestRuntime.isEnabled { return false }
+        return keychain.get(key: Key.enabled) == "true"
+    }
+
+    var isBiometricEnabled: Bool {
+        keychain.get(key: Key.biometric) == "true"
+    }
+
+    var biometricAvailable: Bool {
+        LAContext().canEvaluatePolicy(.deviceOwnerAuthenticationWithBiometrics, error: nil)
+    }
+
+    var biometryType: LABiometryType {
+        let ctx = LAContext()
+        _ = ctx.canEvaluatePolicy(.deviceOwnerAuthenticationWithBiometrics, error: nil)
+        return ctx.biometryType
+    }
+
+    // MARK: - Enable / disable (settings)
+
+    func enable(pin: String, useBiometrics: Bool) {
+        _ = keychain.save(key: Key.pinHash, value: Self.hash(pin))
+        _ = keychain.save(key: Key.biometric, value: useBiometrics ? "true" : "false")
+        _ = keychain.save(key: Key.enabled, value: "true")
+        objectWillChange.send()
+    }
+
+    func disable() {
+        _ = keychain.delete(key: Key.enabled)
+        _ = keychain.delete(key: Key.pinHash)
+        _ = keychain.delete(key: Key.biometric)
+        isLocked = false
+        objectWillChange.send()
+    }
+
+    func setBiometric(_ on: Bool) {
+        _ = keychain.save(key: Key.biometric, value: on ? "true" : "false")
+        objectWillChange.send()
+    }
+
+    // MARK: - Lock / unlock
+
+    /// Arm the lock when leaving the foreground so returning requires re-auth.
+    /// Called on scenePhase `.background`; the lock screen then also serves as
+    /// the app-switcher privacy cover.
+    func lockOnBackground() {
+        if isEnabled { isLocked = true }
+    }
+
+    /// Unlock on a correct PIN (constant-time compare).
+    @discardableResult
+    func unlock(withPIN pin: String) -> Bool {
+        guard let stored = keychain.get(key: Key.pinHash) else { return false }
+        let ok = Self.constantTimeEquals(Self.hash(pin), stored)
+        if ok { isLocked = false }
+        return ok
+    }
+
+    /// Unlock via Face ID / Touch ID.
+    func unlockWithBiometrics() async -> Bool {
+        let ctx = LAContext()
+        guard ctx.canEvaluatePolicy(.deviceOwnerAuthenticationWithBiometrics, error: nil) else { return false }
+        do {
+            let ok = try await ctx.evaluatePolicy(
+                .deviceOwnerAuthenticationWithBiometrics,
+                localizedReason: String(localized: "Unlock honeyDue"))
+            if ok { isLocked = false }
+            return ok
+        } catch {
+            return false
+        }
+    }
+
+    /// Called on logout — never leave the login screen covered.
+    func clearLockState() {
+        isLocked = false
+    }
+
+    // MARK: - PIN hashing
+
+    private static func hash(_ pin: String) -> String {
+        SHA256.hash(data: Data(pin.utf8)).map { String(format: "%02x", $0) }.joined()
+    }
+
+    private static func constantTimeEquals(_ a: String, _ b: String) -> Bool {
+        let ab = Array(a.utf8), bb = Array(b.utf8)
+        guard ab.count == bb.count else { return false }
+        var diff: UInt8 = 0
+        for i in 0..<ab.count { diff |= ab[i] ^ bb[i] }
+        return diff == 0
+    }
+}
diff --git a/iosApp/iosApp/AppLock/AppLockSettingsView.swift b/iosApp/iosApp/AppLock/AppLockSettingsView.swift
new file mode 100644
index 0000000..1a3aef4
--- /dev/null
+++ b/iosApp/iosApp/AppLock/AppLockSettingsView.swift
@@ -0,0 +1,165 @@
+import SwiftUI
+
+/// Settings screen to enable/disable App Lock, choose biometrics, and set/change
+/// the PIN. Reached from the Profile/Settings screen.
+struct AppLockSettingsView: View {
+    @ObservedObject private var lock = AppLockManager.shared
+    @State private var showPinSetup = false
+    @State private var pendingDisable = false
+
+    var body: some View {
+        Form {
+            Section {
+                Toggle(isOn: Binding(
+                    get: { lock.isEnabled },
+                    set: { newValue in
+                        if newValue {
+                            showPinSetup = true        // ask for a PIN before enabling
+                        } else {
+                            pendingDisable = true
+                        }
+                    }
+                )) {
+                    Label("Require unlock", systemImage: "lock.fill")
+                        .foregroundColor(Color.appTextPrimary)
+                }
+                .tint(Color.appPrimary)
+            } footer: {
+                Text("Lock honeyDue when you leave the app. You'll unlock with \(lock.biometricAvailable ? "Face ID / Touch ID or " : "")your PIN.")
+            }
+            .listRowBackground(Color.appBackgroundSecondary)
+
+            if lock.isEnabled {
+                Section {
+                    if lock.biometricAvailable {
+                        Toggle(isOn: Binding(
+                            get: { lock.isBiometricEnabled },
+                            set: { lock.setBiometric($0) }
+                        )) {
+                            Label(lock.biometryType == .faceID ? "Use Face ID" : "Use Touch ID",
+                                  systemImage: lock.biometryType == .faceID ? "faceid" : "touchid")
+                                .foregroundColor(Color.appTextPrimary)
+                        }
+                        .tint(Color.appPrimary)
+                    }
+                    Button {
+                        showPinSetup = true
+                    } label: {
+                        Label("Change PIN", systemImage: "number")
+                            .foregroundColor(Color.appPrimary)
+                    }
+                }
+                .listRowBackground(Color.appBackgroundSecondary)
+            }
+        }
+        .listStyle(.plain)
+        .scrollContentBackground(.hidden)
+        .background(Color.appBackgroundPrimary)
+        .navigationTitle("App Lock")
+        .navigationBarTitleDisplayMode(.inline)
+        .sheet(isPresented: $showPinSetup) {
+            PinSetupView { pin in
+                lock.enable(pin: pin, useBiometrics: lock.biometricAvailable ? lock.isBiometricEnabled || !lock.isEnabled : false)
+                showPinSetup = false
+            } onCancel: {
+                showPinSetup = false
+            }
+        }
+        .alert("Turn off App Lock?", isPresented: $pendingDisable) {
+            Button("Turn Off", role: .destructive) { lock.disable() }
+            Button("Cancel", role: .cancel) {}
+        } message: {
+            Text("Your PIN will be removed and honeyDue will no longer lock.")
+        }
+    }
+}
+
+/// Two-step PIN entry (enter, then confirm). Calls onDone with the PIN on match.
+private struct PinSetupView: View {
+    var onDone: (String) -> Void
+    var onCancel: () -> Void
+
+    @State private var first = ""
+    @State private var confirm = ""
+    @State private var confirming = false
+    @State private var mismatch = false
+    @FocusState private var focused: Bool
+
+    private let pinLength = AppLockManager.pinLength
+
+    var body: some View {
+        NavigationStack {
+            VStack(spacing: 24) {
+                Text(confirming ? "Re-enter your PIN" : "Choose a \(pinLength)-digit PIN")
+                    .font(.system(size: 18, weight: .semibold, design: .rounded))
+                    .foregroundColor(Color.appTextPrimary)
+                    .padding(.top, 40)
+
+                HStack(spacing: 16) {
+                    ForEach(0..<pinLength, id: \.self) { i in
+                        Circle()
+                            .strokeBorder(Color.appPrimary, lineWidth: 1.5)
+                            .background(Circle().fill(i < current.count ? Color.appPrimary : Color.clear))
+                            .frame(width: 15, height: 15)
+                    }
+                }
+
+                if mismatch {
+                    Text("PINs didn't match. Try again.")
+                        .font(.caption)
+                        .foregroundColor(Color.appError)
+                }
+
+                // Hidden field drives entry; tap the dots to focus.
+                TextField("", text: bindingForCurrent)
+                    .keyboardType(.numberPad)
+                    .focused($focused)
+                    .opacity(0.02)
+                    .frame(height: 1)
+
+                Spacer()
+            }
+            .padding(.horizontal, 32)
+            .background(Color.appBackgroundPrimary.ignoresSafeArea())
+            .contentShape(Rectangle())
+            .onTapGesture { focused = true }
+            .onAppear { focused = true }
+            .navigationBarTitleDisplayMode(.inline)
+            .toolbar {
+                ToolbarItem(placement: .cancellationAction) {
+                    Button("Cancel") { onCancel() }
+                }
+            }
+        }
+    }
+
+    private var current: String { confirming ? confirm : first }
+
+    private var bindingForCurrent: Binding<String> {
+        confirming
+            ? Binding(get: { confirm }, set: { handleConfirm($0) })
+            : Binding(get: { first }, set: { handleFirst($0) })
+    }
+
+    private func handleFirst(_ v: String) {
+        first = String(v.prefix(pinLength).filter(\.isNumber))
+        if first.count == pinLength {
+            confirming = true
+            mismatch = false
+        }
+    }
+
+    private func handleConfirm(_ v: String) {
+        confirm = String(v.prefix(pinLength).filter(\.isNumber))
+        if confirm.count == pinLength {
+            if confirm == first {
+                onDone(first)
+            } else {
+                mismatch = true
+                first = ""
+                confirm = ""
+                confirming = false
+            }
+        }
+    }
+}
diff --git a/iosApp/iosApp/AppLock/LockScreenView.swift b/iosApp/iosApp/AppLock/LockScreenView.swift
new file mode 100644
index 0000000..3be7be9
--- /dev/null
+++ b/iosApp/iosApp/AppLock/LockScreenView.swift
@@ -0,0 +1,130 @@
+import SwiftUI
+import LocalAuthentication
+
+/// Full-screen lock overlay shown above the authenticated app when AppLock is
+/// armed. Auto-prompts biometrics (if enabled) and offers a numeric PIN.
+struct LockScreenView: View {
+    @ObservedObject private var lock = AppLockManager.shared
+    @State private var pin = ""
+    @State private var shake = false
+    @State private var biometricDismissed = false
+
+    private var showKeypad: Bool { biometricDismissed || !lock.isBiometricEnabled }
+
+    var body: some View {
+        ZStack {
+            Color.appBackgroundPrimary.ignoresSafeArea()
+
+            VStack(spacing: 28) {
+                Spacer()
+
+                Image(systemName: "lock.fill")
+                    .font(.system(size: 44))
+                    .foregroundColor(Color.appPrimary)
+
+                Text("honeyDue is locked")
+                    .font(.system(size: 22, weight: .bold, design: .rounded))
+                    .foregroundColor(Color.appTextPrimary)
+
+                // PIN progress dots
+                HStack(spacing: 16) {
+                    ForEach(0..<AppLockManager.pinLength, id: \.self) { i in
+                        Circle()
+                            .strokeBorder(Color.appPrimary, lineWidth: 1.5)
+                            .background(Circle().fill(i < pin.count ? Color.appPrimary : Color.clear))
+                            .frame(width: 15, height: 15)
+                    }
+                }
+                .offset(x: shake ? -10 : 0)
+                .animation(.default, value: shake)
+
+                if showKeypad {
+                    keypad
+                }
+
+                if lock.isBiometricEnabled {
+                    Button {
+                        Task { await tryBiometric() }
+                    } label: {
+                        Label(biometricLabel, systemImage: biometricIcon)
+                            .font(.system(size: 15, weight: .semibold))
+                            .foregroundColor(Color.appPrimary)
+                    }
+                    .padding(.top, 4)
+                }
+
+                Spacer()
+            }
+            .padding(.horizontal, 32)
+        }
+        .task {
+            if lock.isBiometricEnabled { await tryBiometric() }
+        }
+    }
+
+    // MARK: - Keypad
+
+    private var keypad: some View {
+        let rows: [[String]] = [["1", "2", "3"], ["4", "5", "6"], ["7", "8", "9"], ["", "0", "<"]]
+        return VStack(spacing: 16) {
+            ForEach(rows.indices, id: \.self) { r in
+                HStack(spacing: 28) {
+                    ForEach(rows[r], id: \.self) { key in
+                        keyButton(key)
+                    }
+                }
+            }
+        }
+    }
+
+    @ViewBuilder
+    private func keyButton(_ key: String) -> some View {
+        if key.isEmpty {
+            Color.clear.frame(width: 72, height: 72)
+        } else if key == "<" {
+            Button { if !pin.isEmpty { pin.removeLast() } } label: {
+                Image(systemName: "delete.left")
+                    .font(.system(size: 22))
+                    .foregroundColor(Color.appTextPrimary)
+                    .frame(width: 72, height: 72)
+            }
+        } else {
+            Button { addDigit(key) } label: {
+                Text(key)
+                    .font(.system(size: 28, weight: .medium, design: .rounded))
+                    .foregroundColor(Color.appTextPrimary)
+                    .frame(width: 72, height: 72)
+                    .background(Circle().fill(Color.appBackgroundSecondary))
+            }
+        }
+    }
+
+    private func addDigit(_ d: String) {
+        guard pin.count < AppLockManager.pinLength else { return }
+        pin.append(d)
+        if pin.count == AppLockManager.pinLength {
+            if lock.unlock(withPIN: pin) {
+                pin = ""
+            } else {
+                shake.toggle()
+                let bad = pin
+                pin = ""
+                // brief haptic-ish reset
+                _ = bad
+            }
+        }
+    }
+
+    private func tryBiometric() async {
+        let ok = await lock.unlockWithBiometrics()
+        if !ok { biometricDismissed = true }
+    }
+
+    private var biometricLabel: String {
+        lock.biometryType == .faceID ? String(localized: "Use Face ID") : String(localized: "Use Touch ID")
+    }
+
+    private var biometricIcon: String {
+        lock.biometryType == .faceID ? "faceid" : "touchid"
+    }
+}
diff --git a/iosApp/iosApp/Profile/ProfileTabView.swift b/iosApp/iosApp/Profile/ProfileTabView.swift
index f94a49f..be46e4d 100644
--- a/iosApp/iosApp/Profile/ProfileTabView.swift
+++ b/iosApp/iosApp/Profile/ProfileTabView.swift
@@ -64,6 +64,11 @@ struct ProfileTabView: View {
                 NavigationLink(destination: Text(L10n.Profile.privacy)) {
                     Label(L10n.Profile.privacy, systemImage: "lock.shield")
                 }
+
+                NavigationLink(destination: AppLockSettingsView()) {
+                    Label("App Lock", systemImage: "lock.fill") // i18n-todo: add L10n.Profile.appLock key
+                        .foregroundColor(Color.appTextPrimary)
+                }
             }
             .sectionBackground()
 
diff --git a/iosApp/iosApp/RootView.swift b/iosApp/iosApp/RootView.swift
index 01dff5f..8508725 100644
--- a/iosApp/iosApp/RootView.swift
+++ b/iosApp/iosApp/RootView.swift
@@ -124,6 +124,9 @@ class AuthenticationManager: ObservableObject {
         isAuthenticated = false
         isVerified = false
 
+        // Never leave the login screen covered by the app-lock overlay.
+        AppLockManager.shared.clearLockState()
+
         // Note: We don't reset onboarding state on logout
         // so returning users go to login screen, not onboarding
 
@@ -143,6 +146,7 @@ struct RootView: View {
     @EnvironmentObject private var themeManager: ThemeManager
     @StateObject private var authManager = AuthenticationManager.shared
     @StateObject private var onboardingState = OnboardingState.shared
+    @StateObject private var appLock = AppLockManager.shared
     @State private var refreshID = UUID()
     @Binding var deepLinkResetToken: String?
 
@@ -202,7 +206,17 @@ struct RootView: View {
             Color.clear
                 .frame(width: 1, height: 1)
                 .accessibilityIdentifier("ui.app.ready")
+
+            // App-lock overlay — covers everything for an authenticated user
+            // when the lock is armed. Sits above all auth states.
+            if appLock.isLocked && authManager.isAuthenticated && authManager.isVerified {
+                LockScreenView()
+                    .transition(.opacity)
+                    .zIndex(10)
+                    .accessibilityIdentifier("ui.app.locked")
+            }
         }
+        .animation(.easeInOut(duration: 0.2), value: appLock.isLocked)
         .task {
             // Trigger auth check here, after iOSApp.init() has completed
             // DataManager.initialize(). This avoids the race condition where
diff --git a/iosApp/iosApp/iOSApp.swift b/iosApp/iosApp/iOSApp.swift
index 565f0c1..d21884e 100644
--- a/iosApp/iosApp/iOSApp.swift
+++ b/iosApp/iosApp/iOSApp.swift
@@ -130,6 +130,10 @@ struct iOSApp: App {
                             }
                         }
                     } else if newPhase == .background {
+                        // Arm the app-lock so returning requires re-auth; the lock
+                        // screen also serves as the app-switcher privacy cover.
+                        AppLockManager.shared.lockOnBackground()
+
                         // Refresh widget when app goes to background
                         WidgetCenter.shared.reloadAllTimelines()