diff --git a/backend/internal/usersignup/service.go b/backend/internal/usersignup/service.go
index 0a26f6b3..1d2ffed9 100644
--- a/backend/internal/usersignup/service.go
+++ b/backend/internal/usersignup/service.go
@@ -148,6 +148,13 @@ func (s *Service) SignUpInitialAdmin(ctx context.Context, config *appconfig.AppC
 		tx.Rollback()
 	}()
 
+	// We lock the users table to prevent concurrent initial admin setups from racing to create the first user
+	// This is only necessary for Postgres, since SQLite serializes all writes anyway
+	if err := lockInitialAdminSetup(ctx, tx); err != nil {
+		return model.User{}, "", err
+	}
+
+	// Reject setup when a committed user already exists
 	setupCompleted, err := s.isInitialAdminSetupCompleted(ctx, tx)
 	if err != nil {
 		return model.User{}, "", err
@@ -156,6 +163,7 @@ func (s *Service) SignUpInitialAdmin(ctx context.Context, config *appconfig.AppC
 		return model.User{}, "", &common.SetupNotAvailableError{}
 	}
 
+	// Build the first user with administrator privileges
 	userToCreate := dto.UserCreateDto{
 		FirstName:   signUpData.FirstName,
 		LastName:    signUpData.LastName,
@@ -170,6 +178,7 @@ func (s *Service) SignUpInitialAdmin(ctx context.Context, config *appconfig.AppC
 		return model.User{}, "", err
 	}
 
+	// Issue the setup session before committing so failures roll back the transaction
 	token, err := s.signer.GenerateAccessToken(user, authenticationMethodOneTimePassword, config.SessionDuration.AsDurationMinutes())
 	if err != nil {
 		return model.User{}, "", err
@@ -183,6 +192,18 @@ func (s *Service) SignUpInitialAdmin(ctx context.Context, config *appconfig.AppC
 	return user, token, nil
 }
 
+func lockInitialAdminSetup(ctx context.Context, tx *gorm.DB) error {
+	if tx.Name() != "postgres" {
+		return nil
+	}
+
+	if err := tx.WithContext(ctx).Exec("LOCK TABLE users IN SHARE ROW EXCLUSIVE MODE").Error; err != nil {
+		return fmt.Errorf("failed to lock users table for initial admin setup: %w", err)
+	}
+
+	return nil
+}
+
 func (s *Service) IsInitialAdminSetupCompleted(ctx context.Context) (bool, error) {
 	return s.isInitialAdminSetupCompleted(ctx, s.db)
 }
diff --git a/backend/internal/usersignup/service_test.go b/backend/internal/usersignup/service_test.go
index bc3a424b..3af38502 100644
--- a/backend/internal/usersignup/service_test.go
+++ b/backend/internal/usersignup/service_test.go
@@ -118,6 +118,46 @@ func TestSignUpRejectsInvalidToken(t *testing.T) {
 	require.ErrorAs(t, err, &invalidErr)
 }
 
+func TestSignUpInitialAdminCreatesAdmin(t *testing.T) {
+	db := testutils.NewDatabaseForTest(t)
+	svc := newSignupServiceForTest(t, db, fakeUserCreator{user: model.User{Base: model.Base{ID: "new-admin"}}})
+	config := appconfig.NewTestConfig(nil)
+
+	// Complete setup and return the generated administrator session
+	user, accessToken, err := svc.SignUpInitialAdmin(t.Context(), config, signUpDto{Username: "new-admin"})
+	require.NoError(t, err)
+	require.Equal(t, "new-admin", user.ID)
+	require.Equal(t, "access-token", accessToken)
+}
+
+func TestSignUpInitialAdminAllowsRetryAfterFailure(t *testing.T) {
+	db := testutils.NewDatabaseForTest(t)
+	boom := errors.New("could not create initial admin")
+	svc := newSignupServiceForTest(t, db, fakeUserCreator{err: boom})
+	config := appconfig.NewTestConfig(nil)
+
+	// Fail the first setup transaction before it can commit
+	_, _, err := svc.SignUpInitialAdmin(t.Context(), config, signUpDto{Username: "failed-admin"})
+	require.ErrorIs(t, err, boom)
+
+	// Confirm a later setup can complete after the failed transaction rolls back
+	svc.userCreator = fakeUserCreator{user: model.User{Base: model.Base{ID: "new-admin"}}}
+	user, _, err := svc.SignUpInitialAdmin(t.Context(), config, signUpDto{Username: "new-admin"})
+	require.NoError(t, err)
+	require.Equal(t, "new-admin", user.ID)
+}
+
+func TestSignUpInitialAdminRejectsExistingInstallation(t *testing.T) {
+	db := testutils.NewDatabaseForTest(t)
+	require.NoError(t, db.Create(&model.User{Username: "existing-admin", IsAdmin: true}).Error)
+	svc := newSignupServiceForTest(t, db, fakeUserCreator{user: model.User{Base: model.Base{ID: "new-admin"}}})
+
+	// Reject setup when the installation already contains a user
+	_, _, err := svc.SignUpInitialAdmin(t.Context(), appconfig.NewTestConfig(nil), signUpDto{Username: "new-admin"})
+	var setupNotAvailableErr *common.SetupNotAvailableError
+	require.ErrorAs(t, err, &setupNotAvailableErr)
+}
+
 // listAllOptions returns list options that return every token on a single page.
 func listAllOptions() utils.ListRequestOptions {
 	var opts utils.ListRequestOptions
diff --git a/tests/specs/user-signup.spec.ts b/tests/specs/user-signup.spec.ts
index 2871bfc1..7b2129cd 100644
--- a/tests/specs/user-signup.spec.ts
+++ b/tests/specs/user-signup.spec.ts
@@ -81,6 +81,37 @@ test.describe('Initial User Signup', () => {
 		await expect(page.getByText('Set up your passkey')).toBeVisible();
 	});
 
+	test('Initial Signup - concurrent requests create one administrator', async ({ request }) => {
+		await cleanupBackend({ skipSeed: true });
+
+		const requestCount = 20;
+		const responses = await Promise.all(
+			Array.from({ length: requestCount }, (_, index) =>
+				request.post('/api/signup/setup', {
+					data: {
+						username: `race-admin-${index}`,
+						email: `race-admin-${index}@example.invalid`,
+						firstName: 'Race',
+						lastName: `${index}`
+					}
+				})
+			)
+		);
+
+		const successfulResponses = responses.filter((response) => response.status() === 200);
+		const rejectedResponses = responses.filter((response) => response.status() === 404);
+
+		expect(successfulResponses).toHaveLength(1);
+		expect(rejectedResponses).toHaveLength(requestCount - 1);
+		await expect(successfulResponses[0].json()).resolves.toMatchObject({ isAdmin: true });
+
+		const usersResponse = await request.get('/api/users');
+		expect(usersResponse.status()).toBe(200);
+		const users = await usersResponse.json();
+		expect(users.data).toHaveLength(1);
+		expect(users.data[0]).toMatchObject({ isAdmin: true });
+	});
+
 	test('Initial Signup - setup route unavailable after completion', async ({ page }) => {
 		await cleanupBackend();
 		await page.goto('/setup');