diff --git a/apps/backend/src/__tests__/auth.test.ts b/apps/backend/src/__tests__/auth.test.ts
new file mode 100644
index 00000000..81006eea
--- /dev/null
+++ b/apps/backend/src/__tests__/auth.test.ts
@@ -0,0 +1,156 @@
+import cookie from '@fastify/cookie';
+import jwt from '@fastify/jwt';
+import Fastify from 'fastify';
+import { describe, it, expect, beforeEach, afterEach, vi } from 'vitest';
+
+import { authRoutes } from '../routes/auth.js';
+
+import type { PrismaClient } from '@prisma/client';
+
+const mockPrisma = {
+  user: {
+    upsert: vi.fn(),
+    findUnique: vi.fn(),
+  },
+  oAuthToken: {
+    upsert: vi.fn(),
+  },
+};
+
+const originalEnv = process.env;
+const originalFetch = global.fetch;
+
+async function buildApp() {
+  const app = Fastify();
+  await app.register(jwt, { secret: 'test-secret' });
+  await app.register(cookie);
+  app.decorate('prisma', mockPrisma as unknown as PrismaClient);
+  app.decorate('authenticate', async () => {});
+
+  app.register(authRoutes, { prefix: '/auth' });
+  await app.ready();
+  return app;
+}
+
+describe('GET /auth/google/callback', () => {
+  beforeEach(() => {
+    vi.clearAllMocks();
+    process.env = { ...originalEnv };
+    process.env.NODE_ENV = 'test';
+    process.env.PUBLIC_APP_URL = 'http://localhost:3000';
+    process.env.BACKEND_URL = 'http://localhost:3001';
+    process.env.MOBILE_REDIRECT_URI = 'devcard://auth';
+    process.env.GOOGLE_CLIENT_ID = 'test-google-id';
+    process.env.GOOGLE_CLIENT_SECRET = 'test-google-secret';
+    process.env.ENCRYPTION_KEY = '12345678901234567890123456789012';
+
+    global.fetch = vi.fn();
+  });
+
+  afterEach(() => {
+    process.env = originalEnv;
+    global.fetch = originalFetch;
+  });
+
+  it('persists Google OAuthToken upon successful login', async () => {
+    const mockUser = { id: 'user-123', username: 'testuser' };
+    mockPrisma.user.upsert.mockResolvedValue(mockUser);
+    mockPrisma.oAuthToken.upsert.mockResolvedValue({});
+
+    (global.fetch as any)
+      .mockResolvedValueOnce({
+        json: vi.fn().mockResolvedValue({
+          access_token: 'fake-google-token',
+          scope: 'openid email profile',
+        }),
+      }) // tokenRes
+      .mockResolvedValueOnce({
+        json: vi.fn().mockResolvedValue({
+          id: 'google-id-123',
+          email: 'test@gmail.com',
+          name: 'Test User',
+          picture: 'https://avatar.com/test',
+        }),
+      }); // userRes
+
+    const app = await buildApp();
+
+    const res = await app.inject({
+      method: 'GET',
+      url: '/auth/google/callback?code=fake-code&state=fake-state',
+      cookies: {
+        oauth_state: 'fake-state',
+      },
+    });
+
+    expect(res.statusCode).toBe(302);
+    expect(res.headers.location).toBe('http://localhost:3000/dashboard');
+
+    // Verify user upsert
+    expect(mockPrisma.user.upsert).toHaveBeenCalledWith(
+      expect.objectContaining({
+        where: { provider_providerId: { provider: 'google', providerId: 'google-id-123' } },
+      })
+    );
+
+    // Verify oAuthToken upsert
+    expect(mockPrisma.oAuthToken.upsert).toHaveBeenCalledWith(
+      expect.objectContaining({
+        where: { userId_platform: { userId: 'user-123', platform: 'google' } },
+        create: expect.objectContaining({
+          platform: 'google',
+          scopes: 'openid email profile',
+        }),
+      })
+    );
+
+    await app.close();
+  });
+
+  it('allows authentication to succeed even if token persistence fails', async () => {
+    const mockUser = { id: 'user-123', username: 'testuser' };
+    mockPrisma.user.upsert.mockResolvedValue(mockUser);
+
+    // Simulate upsert failure
+    mockPrisma.oAuthToken.upsert.mockRejectedValue(new Error('DB Error'));
+
+    (global.fetch as any)
+      .mockResolvedValueOnce({
+        json: vi.fn().mockResolvedValue({
+          access_token: 'fake-google-token',
+        }),
+      })
+      .mockResolvedValueOnce({
+        json: vi.fn().mockResolvedValue({
+          id: 'google-id-123',
+          email: 'test@gmail.com',
+          name: 'Test User',
+        }),
+      });
+
+    const app = await buildApp();
+
+    // Spy on app.log.error to ensure it gets logged
+    const logSpy = vi.spyOn(app.log, 'error');
+
+    const res = await app.inject({
+      method: 'GET',
+      url: '/auth/google/callback?code=fake-code&state=fake-state',
+      cookies: {
+        oauth_state: 'fake-state',
+      },
+    });
+
+    // Should still succeed and redirect to dashboard
+    expect(res.statusCode).toBe(302);
+    expect(res.headers.location).toBe('http://localhost:3000/dashboard');
+
+    // Verify the error was logged
+    expect(logSpy).toHaveBeenCalledWith(
+      expect.objectContaining({ userId: 'user-123' }),
+      'Failed to persist Google OAuth token — authentication proceeds'
+    );
+
+    await app.close();
+  });
+});
diff --git a/apps/backend/src/routes/auth.ts b/apps/backend/src/routes/auth.ts
index cffebea7..89387ea5 100644
--- a/apps/backend/src/routes/auth.ts
+++ b/apps/backend/src/routes/auth.ts
@@ -232,6 +232,19 @@ export async function authRoutes(app: FastifyInstance): Promise<void> {
         },
       });
 
+      try {
+        const encryptedToken = encrypt(tokenData.access_token);
+        const scopes = tokenData.scope || 'openid email profile';
+        const platform = 'google' as const;
+        await app.prisma.oAuthToken.upsert({
+          where: { userId_platform: { userId: user.id, platform } },
+          update: { accessToken: encryptedToken, scopes },
+          create: { userId: user.id, platform, accessToken: encryptedToken, scopes },
+        });
+      } catch (err) {
+        app.log.error({ err, userId: user.id }, 'Failed to persist Google OAuth token — authentication proceeds');
+      }
+
       const token = app.jwt.sign({ id: user.id, username: user.username }, { expiresIn: '30d' });
 
       if (request.query.state?.startsWith('mobile_')) {