diff --git a/.changeset/add-lock-app-functionality.md b/.changeset/add-lock-app-functionality.md
new file mode 100644
index 0000000..004977d
--- /dev/null
+++ b/.changeset/add-lock-app-functionality.md
@@ -0,0 +1,5 @@
+---
+"think-app": patch
+---
+
+Add lock app functionality to settings page
diff --git a/app/src/pages/SettingsPage.tsx b/app/src/pages/SettingsPage.tsx
index befa485..a5b85b2 100644
--- a/app/src/pages/SettingsPage.tsx
+++ b/app/src/pages/SettingsPage.tsx
@@ -4,7 +4,7 @@ import { Card, CardHeader, CardTitle, CardContent } from "@/components/ui/card";
 import { Button } from "@/components/ui/button";
 import { Input } from "@/components/ui/input";
 import { Progress } from "@/components/ui/progress";
-import { Check, Circle, Loader2, Monitor, Sun, Moon, AlertTriangle } from "lucide-react";
+import { Check, Circle, Loader2, Monitor, Sun, Moon, AlertTriangle, LogOut } from "lucide-react";
 import { Theme, setTheme, getTheme } from "@/hooks/useSystemTheme";
 import { apiFetch } from "@/lib/api";
 import { ModelSelector } from "@/components/ModelSelector";
@@ -67,6 +67,10 @@ export default function SettingsPage({ onNameChange }: SettingsPageProps) {
   const [staleEmbeddingsCount, setStaleEmbeddingsCount] = useState(0);
   const [showReembedDialog, setShowReembedDialog] = useState(false);
 
+  // Lock app state
+  const [showLockDialog, setShowLockDialog] = useState(false);
+  const [locking, setLocking] = useState(false);
+
   // Use the reembed job hook for background re-embedding
   const reembedJob = useReembedJob({
     onComplete: (processed, failed) => {
@@ -309,6 +313,20 @@ export default function SettingsPage({ onNameChange }: SettingsPageProps) {
     setTheme(newTheme);
   };
 
+  const handleLock = async () => {
+    setLocking(true);
+    try {
+      const res = await apiFetch("/api/auth/logout", { method: "POST" });
+      if (res.ok) {
+        window.location.reload();
+      }
+    } catch (err) {
+      console.error("Failed to lock:", err);
+    } finally {
+      setLocking(false);
+    }
+  };
+
   // Progress from the job hook
   const progressPercent = reembedJob.progress;
 
@@ -567,6 +585,26 @@ export default function SettingsPage({ onNameChange }: SettingsPageProps) {
             </Button>
           </CardContent>
         </Card>
+
+        {/* Security Section */}
+        <Card>
+          <CardHeader>
+            <CardTitle className="text-base">Security</CardTitle>
+          </CardHeader>
+          <CardContent>
+            <Button
+              variant="outline"
+              onClick={() => setShowLockDialog(true)}
+              className="w-full"
+            >
+              <LogOut className="h-4 w-4 mr-2" />
+              Lock App
+            </Button>
+            <p className="text-xs text-muted-foreground mt-2 text-center">
+              Lock the app and require password to access
+            </p>
+          </CardContent>
+        </Card>
       </div>
 
       {/* Settings change warning dialog - rendered via portal for full-screen overlay */}
@@ -685,6 +723,39 @@ export default function SettingsPage({ onNameChange }: SettingsPageProps) {
         </div>,
         document.body
       )}
+
+      {/* Lock app confirmation dialog */}
+      {showLockDialog && createPortal(
+        <div className="fixed inset-0 bg-black/50 flex items-center justify-center z-[100]">
+          <div className="bg-background border rounded-lg shadow-xl max-w-md w-full mx-4 p-6">
+            <div className="flex items-start gap-3 mb-4">
+              <LogOut className="h-6 w-6 text-muted-foreground flex-shrink-0 mt-0.5" />
+              <div>
+                <h3 className="font-semibold text-lg">Lock App?</h3>
+                <p className="text-sm text-muted-foreground mt-1">
+                  You'll need to enter your password to unlock.
+                </p>
+              </div>
+            </div>
+
+            <div className="flex flex-col gap-2">
+              <Button onClick={handleLock} disabled={locking} className="w-full">
+                {locking && <Loader2 className="h-4 w-4 animate-spin mr-2" />}
+                Lock
+              </Button>
+              <Button
+                variant="ghost"
+                onClick={() => setShowLockDialog(false)}
+                disabled={locking}
+                className="w-full"
+              >
+                Cancel
+              </Button>
+            </div>
+          </div>
+        </div>,
+        document.body
+      )}
     </>
   );
 }
diff --git a/backend/app/db/__init__.py b/backend/app/db/__init__.py
index 5311cdd..b12fab1 100644
--- a/backend/app/db/__init__.py
+++ b/backend/app/db/__init__.py
@@ -1,4 +1,4 @@
-from .core import init_db, is_db_initialized, db_exists, DB_PATH
+from .core import init_db, is_db_initialized, db_exists, reset_db_connection, DB_PATH
 from .crud import (
     create_memory,
     get_memories,
@@ -28,6 +28,7 @@
     "init_db",
     "is_db_initialized",
     "db_exists",
+    "reset_db_connection",
     "DB_PATH",
     "create_memory",
     "get_memories",
diff --git a/backend/app/db/core.py b/backend/app/db/core.py
index b3a19ec..9fc3e95 100644
--- a/backend/app/db/core.py
+++ b/backend/app/db/core.py
@@ -125,6 +125,16 @@ def is_db_initialized() -> bool:
     return _engine is not None
 
 
+def reset_db_connection():
+    """Reset database connection and clear encryption key (logout)."""
+    global _engine, _session_maker, _db_key
+    if _engine is not None:
+        _engine.dispose()
+    _engine = None
+    _session_maker = None
+    _db_key = None
+
+
 def db_exists() -> bool:
     """Check if the database file exists (password was set)."""
     return DB_PATH.exists()
diff --git a/backend/app/main.py b/backend/app/main.py
index 37a1be8..a721446 100644
--- a/backend/app/main.py
+++ b/backend/app/main.py
@@ -61,7 +61,7 @@ async def lifespan(app: FastAPI):
 )
 
 # Paths that don't require unlock
-PUBLIC_PATHS = {"/health", "/api/auth/status", "/api/auth/setup", "/api/auth/unlock"}
+PUBLIC_PATHS = {"/health", "/api/auth/status", "/api/auth/setup", "/api/auth/unlock", "/api/auth/logout"}
 
 
 @app.middleware("http")
diff --git a/backend/app/routes/auth.py b/backend/app/routes/auth.py
index 825b34a..4b0ede2 100644
--- a/backend/app/routes/auth.py
+++ b/backend/app/routes/auth.py
@@ -1,7 +1,7 @@
 from fastapi import APIRouter, HTTPException
 
 from ..config import reload_settings
-from ..db import init_db, is_db_initialized, db_exists
+from ..db import init_db, is_db_initialized, db_exists, reset_db_connection
 from ..services.secrets import derive_db_key, set_api_key, get_api_key, delete_api_key
 from ..schemas import SetPasswordRequest, UnlockRequest, ApiKeyRequest
 
@@ -47,6 +47,13 @@ async def unlock(request: UnlockRequest):
     return {"success": True}
 
 
+@router.post("/auth/logout")
+async def logout():
+    """Lock the database (logout)."""
+    reset_db_connection()
+    return {"success": True}
+
+
 @router.post("/settings/api-key")
 async def save_api_key_endpoint(request: ApiKeyRequest):
     """Save an API key to the encrypted database."""