diff --git a/prisma/migrations/20260304000000_normalize_local_usernames/migration.sql b/prisma/migrations/20260304000000_normalize_local_usernames/migration.sql
new file mode 100644
index 0000000..5fbb0c2
--- /dev/null
+++ b/prisma/migrations/20260304000000_normalize_local_usernames/migration.sql
@@ -0,0 +1,3 @@
+-- Normalize existing local usernames to lowercase
+UPDATE users SET plex_username = LOWER(plex_username) WHERE auth_provider = 'local' AND deleted_at IS NULL;
+UPDATE users SET plex_id = 'local-' || LOWER(SUBSTRING(plex_id FROM 7)) WHERE plex_id LIKE 'local-%' AND plex_id NOT LIKE 'local-%-deleted-%';
diff --git a/src/app/api/auth/admin/login/route.ts b/src/app/api/auth/admin/login/route.ts
index 291bb20..46310c5 100644
--- a/src/app/api/auth/admin/login/route.ts
+++ b/src/app/api/auth/admin/login/route.ts
@@ -38,9 +38,11 @@ export async function POST(request: NextRequest) {
       );
     }
 
+    const normalizedUsername = username.trim().toLowerCase();
+
     // Find user by local admin identifier
     const user = await prisma.user.findUnique({
-      where: { plexId: `local-${username}` },
+      where: { plexId: `local-${normalizedUsername}` },
     });
 
     if (!user) {
diff --git a/src/app/api/setup/complete/route.ts b/src/app/api/setup/complete/route.ts
index 24dd593..637e0da 100644
--- a/src/app/api/setup/complete/route.ts
+++ b/src/app/api/setup/complete/route.ts
@@ -140,14 +140,15 @@ export async function POST(request: NextRequest) {
         );
       }
 
+      const normalizedAdminUsername = admin.username.trim().toLowerCase();
       const hashedPassword = await bcrypt.hash(admin.password, 10);
       const encryptionService = getEncryptionService();
       const encryptedPassword = encryptionService.encrypt(hashedPassword);
 
       adminUser = await prisma.user.create({
         data: {
-          plexId: `local-${admin.username}`,
-          plexUsername: admin.username,
+          plexId: `local-${normalizedAdminUsername}`,
+          plexUsername: normalizedAdminUsername,
           plexEmail: null,
           role: 'admin',
           isSetupAdmin: true, // Mark as setup admin - role cannot be changed
diff --git a/src/lib/services/auth/LocalAuthProvider.ts b/src/lib/services/auth/LocalAuthProvider.ts
index 1fbc929..c3dddb6 100644
--- a/src/lib/services/auth/LocalAuthProvider.ts
+++ b/src/lib/services/auth/LocalAuthProvider.ts
@@ -54,10 +54,12 @@ export class LocalAuthProvider implements IAuthProvider {
         return { success: false, error: 'Username and password required' };
       }
 
+      const normalizedUsername = username.trim().toLowerCase();
+
       // Find user (exclude soft-deleted users)
       const user = await prisma.user.findFirst({
         where: {
-          plexUsername: username,
+          plexUsername: normalizedUsername,
           authProvider: 'local',
           deletedAt: null, // Exclude soft-deleted users
         },
@@ -144,9 +146,10 @@ export class LocalAuthProvider implements IAuthProvider {
   async register(params: RegisterParams): Promise<AuthResult> {
     try {
       const { username, password } = params;
+      const normalizedUsername = username?.trim().toLowerCase();
 
       // Validate
-      if (!username || username.length < 3) {
+      if (!normalizedUsername || normalizedUsername.length < 3) {
         return { success: false, error: 'Username must be at least 3 characters' };
       }
 
@@ -167,7 +170,7 @@ export class LocalAuthProvider implements IAuthProvider {
       // Check username uniqueness (only among non-deleted users)
       const existing = await prisma.user.findFirst({
         where: {
-          plexUsername: username,
+          plexUsername: normalizedUsername,
           authProvider: 'local',
           deletedAt: null, // Allow reuse of usernames from deleted accounts
         },
@@ -194,8 +197,8 @@ export class LocalAuthProvider implements IAuthProvider {
       // Create user
       const user = await prisma.user.create({
         data: {
-          plexId: `local-${username}`,
-          plexUsername: username,
+          plexId: `local-${normalizedUsername}`,
+          plexUsername: normalizedUsername,
           authToken: encryptedHash,
           authProvider: 'local',
           role: isFirstUser ? 'admin' : 'user',
diff --git a/tests/services/auth/local-auth-provider.test.ts b/tests/services/auth/local-auth-provider.test.ts
index ff7a16e..f1e0a11 100644
--- a/tests/services/auth/local-auth-provider.test.ts
+++ b/tests/services/auth/local-auth-provider.test.ts
@@ -167,6 +167,31 @@ describe('LocalAuthProvider', () => {
     expect(result.error).toMatch(/invalid username or password/i);
   });
 
+  it('normalizes username to lowercase on login', async () => {
+    prismaMock.user.findFirst.mockResolvedValue({
+      id: 'user-ci',
+      plexId: 'local-admin',
+      plexUsername: 'admin',
+      role: 'admin',
+      authProvider: 'local',
+      authToken: 'enc:hash',
+      registrationStatus: 'approved',
+      deletedAt: null,
+    });
+    prismaMock.user.update.mockResolvedValue({});
+    bcryptCompare.mockResolvedValue(true);
+
+    const { LocalAuthProvider } = await import('@/lib/services/auth/LocalAuthProvider');
+    const provider = new LocalAuthProvider();
+    await provider.handleCallback({ username: 'Admin', password: 'pass' });
+
+    expect(prismaMock.user.findFirst).toHaveBeenCalledWith(
+      expect.objectContaining({
+        where: expect.objectContaining({ plexUsername: 'admin' }),
+      })
+    );
+  });
+
   it('blocks registration when disabled', async () => {
     configMock.get.mockResolvedValueOnce('false');
 
@@ -237,6 +262,51 @@ describe('LocalAuthProvider', () => {
     expect(result.error).toContain('Username already taken');
   });
 
+  it('stores lowercase username and plexId on registration', async () => {
+    configMock.get.mockResolvedValueOnce('true'); // registration enabled
+    configMock.get.mockResolvedValueOnce('false'); // no admin approval
+    prismaMock.user.findFirst.mockResolvedValue(null);
+    prismaMock.user.count.mockResolvedValue(1);
+    prismaMock.user.create.mockResolvedValue({
+      id: 'user-ci2',
+      plexId: 'local-myuser',
+      plexUsername: 'myuser',
+      role: 'user',
+    });
+    bcryptHash.mockResolvedValue('hash');
+
+    const { LocalAuthProvider } = await import('@/lib/services/auth/LocalAuthProvider');
+    const provider = new LocalAuthProvider();
+    await provider.register({ username: 'MyUser', password: 'password123' });
+
+    expect(prismaMock.user.create).toHaveBeenCalledWith(
+      expect.objectContaining({
+        data: expect.objectContaining({
+          plexId: 'local-myuser',
+          plexUsername: 'myuser',
+        }),
+      })
+    );
+  });
+
+  it('rejects duplicate username case-insensitively on registration', async () => {
+    configMock.get.mockResolvedValueOnce('true'); // registration enabled
+    prismaMock.user.findFirst.mockResolvedValue({ id: 'user-existing' });
+
+    const { LocalAuthProvider } = await import('@/lib/services/auth/LocalAuthProvider');
+    const provider = new LocalAuthProvider();
+    const result = await provider.register({ username: 'User', password: 'password123' });
+
+    expect(result.success).toBe(false);
+    expect(result.error).toContain('Username already taken');
+    // The lookup should use the lowercased username
+    expect(prismaMock.user.findFirst).toHaveBeenCalledWith(
+      expect.objectContaining({
+        where: expect.objectContaining({ plexUsername: 'user' }),
+      })
+    );
+  });
+
   it('creates admin user on first registration', async () => {
     configMock.get.mockResolvedValueOnce('true'); // registration enabled
     configMock.get.mockResolvedValueOnce('false'); // no admin approval