diff --git a/internal/models/user.go b/internal/models/user.go
index dd66fb3..9a48f09 100644
--- a/internal/models/user.go
+++ b/internal/models/user.go
@@ -135,6 +135,61 @@ func FindUserByUsername(
 	return user, nil
 }
 
+// CreateFirstUser atomically checks that no users exist and inserts the admin user.
+// Returns nil, nil if a user already exists (setup already completed).
+func CreateFirstUser(
+	ctx context.Context,
+	db *database.Database,
+	username, passwordHash string,
+) (*User, error) {
+	tx, err := db.BeginTx(ctx, nil)
+	if err != nil {
+		return nil, fmt.Errorf("beginning transaction: %w", err)
+	}
+
+	defer func() { _ = tx.Rollback() }()
+
+	// Check if any user exists within the transaction.
+	var count int
+
+	err = tx.QueryRowContext(ctx, "SELECT COUNT(*) FROM users").Scan(&count)
+	if err != nil {
+		return nil, fmt.Errorf("checking user count: %w", err)
+	}
+
+	if count > 0 {
+		return nil, nil //nolint:nilnil // nil,nil signals setup already completed
+	}
+
+	result, err := tx.ExecContext(ctx,
+		"INSERT INTO users (username, password_hash) VALUES (?, ?)",
+		username, passwordHash,
+	)
+	if err != nil {
+		return nil, fmt.Errorf("inserting user: %w", err)
+	}
+
+	err = tx.Commit()
+	if err != nil {
+		return nil, fmt.Errorf("committing transaction: %w", err)
+	}
+
+	insertID, err := result.LastInsertId()
+	if err != nil {
+		return nil, fmt.Errorf("getting last insert id: %w", err)
+	}
+
+	user := NewUser(db)
+	user.ID = insertID
+
+	err = user.Reload(ctx)
+	if err != nil {
+		return nil, fmt.Errorf("reloading user: %w", err)
+	}
+
+	return user, nil
+}
+
 // UserExists checks if any user exists in the database.
 func UserExists(ctx context.Context, db *database.Database) (bool, error) {
 	var count int
diff --git a/internal/service/auth/auth.go b/internal/service/auth/auth.go
index 94447e9..aa83128 100644
--- a/internal/service/auth/auth.go
+++ b/internal/service/auth/auth.go
@@ -163,34 +163,27 @@ func (svc *Service) IsSetupRequired(ctx context.Context) (bool, error) {
 }
 
 // CreateUser creates the initial admin user.
+// It uses a DB transaction to atomically check that no users exist and insert
+// the new admin user, preventing race conditions from concurrent setup requests.
 func (svc *Service) CreateUser(
 	ctx context.Context,
 	username, password string,
 ) (*models.User, error) {
-	// Check if user already exists
-	exists, err := models.UserExists(ctx, svc.db)
-	if err != nil {
-		return nil, fmt.Errorf("failed to check if user exists: %w", err)
-	}
-
-	if exists {
-		return nil, ErrUserExists
-	}
-
-	// Hash password
+	// Hash password before starting transaction.
 	hash, err := svc.HashPassword(password)
 	if err != nil {
 		return nil, fmt.Errorf("failed to hash password: %w", err)
 	}
 
-	// Create user
-	user := models.NewUser(svc.db)
-	user.Username = username
-	user.PasswordHash = hash
-
-	err = user.Save(ctx)
+	// Use a transaction so the "no users exist" check and the insert are atomic.
+	// SQLite serializes write transactions, so concurrent requests will block here.
+	user, err := models.CreateFirstUser(ctx, svc.db, username, hash)
 	if err != nil {
-		return nil, fmt.Errorf("failed to save user: %w", err)
+		return nil, fmt.Errorf("failed to create user: %w", err)
+	}
+
+	if user == nil {
+		return nil, ErrUserExists
 	}
 
 	svc.log.Info("user created", "username", username)
diff --git a/internal/service/auth/auth_test.go b/internal/service/auth/auth_test.go
index 9ca9430..3d1b8a3 100644
--- a/internal/service/auth/auth_test.go
+++ b/internal/service/auth/auth_test.go
@@ -2,6 +2,7 @@ package auth_test
 
 import (
 	"context"
+	"fmt"
 	"net/http"
 	"net/http/httptest"
 	"path/filepath"
@@ -270,6 +271,52 @@ func TestCreateUser(testingT *testing.T) {
 	})
 }
 
+func TestCreateUserRaceCondition(testingT *testing.T) {
+	testingT.Parallel()
+
+	testingT.Run("concurrent setup requests create only one user", func(t *testing.T) {
+		t.Parallel()
+
+		svc, cleanup := setupTestService(t)
+		defer cleanup()
+
+		const goroutines = 10
+
+		results := make(chan error, goroutines)
+		start := make(chan struct{})
+
+		for i := range goroutines {
+			go func(idx int) {
+				<-start // Wait for all goroutines to be ready
+
+				_, err := svc.CreateUser(
+					context.Background(),
+					fmt.Sprintf("admin%d", idx),
+					"password123456",
+				)
+				results <- err
+			}(i)
+		}
+
+		// Release all goroutines simultaneously
+		close(start)
+
+		var successes, failures int
+		for range goroutines {
+			err := <-results
+			if err == nil {
+				successes++
+			} else {
+				assert.ErrorIs(t, err, auth.ErrUserExists)
+				failures++
+			}
+		}
+
+		assert.Equal(t, 1, successes, "exactly one goroutine should succeed")
+		assert.Equal(t, goroutines-1, failures, "all other goroutines should fail with ErrUserExists")
+	})
+}
+
 func TestAuthenticate(testingT *testing.T) {
 	testingT.Parallel()