diff --git a/client.go b/client.go
index 997e355e..f496f966 100644
--- a/client.go
+++ b/client.go
@@ -261,7 +261,18 @@ func NewClient(ctx context.Context, cc *ClientConfig) (*Client, error) {
 			}
 			cc.HTTPClient = client
 		} else {
-			cc.HTTPClient = &http.Client{}
+			// If credentials are provided for Gemini API, create an authenticated HTTP client
+			if cc.Credentials != nil {
+				client, err := httptransport.NewClient(&httptransport.Options{
+					Credentials: cc.Credentials,
+				})
+				if err != nil {
+					return nil, fmt.Errorf("failed to create HTTP client: %w", err)
+				}
+				cc.HTTPClient = client
+			} else {
+				cc.HTTPClient = &http.Client{}
+			}
 		}
 	}
 
diff --git a/client_test.go b/client_test.go
index 1ca432e2..14be46bf 100644
--- a/client_test.go
+++ b/client_test.go
@@ -433,6 +433,82 @@ func TestNewClient(t *testing.T) {
 
 }
 
+func TestCustomCredentialsWithGeminiAPI(t *testing.T) {
+	// This test verifies that custom credentials are properly used when creating a client
+	// with the Gemini API backend.
+	ctx := context.Background()
+
+	// Create mock credentials
+	mockCreds := &auth.Credentials{}
+
+	// Test case: Gemini API with custom credentials
+	t.Run("GeminiAPI with custom credentials", func(t *testing.T) {
+		client, err := NewClient(ctx, &ClientConfig{
+			Backend:     BackendGeminiAPI,
+			APIKey:      "test-api-key",
+			Credentials: mockCreds,
+		})
+		if err != nil {
+			t.Fatalf("Expected no error, got %v", err)
+		}
+
+		// Verify the credentials were correctly passed to the client config
+		if client.clientConfig.Credentials != mockCreds {
+			t.Errorf("Credentials were not properly set in client config")
+		}
+
+		// Verify that HTTPClient is not nil (it should have been created)
+		if client.clientConfig.HTTPClient == nil {
+			t.Errorf("Expected HTTPClient to be created, got nil")
+		}
+	})
+
+	// Test case: Custom HTTP options with Gemini API
+	t.Run("GeminiAPI with custom HTTP options", func(t *testing.T) {
+		customOptions := HTTPOptions{
+			APIVersion: "custom-version",
+			BaseURL:    "https://custom-url.example.com/",
+		}
+
+		client, err := NewClient(ctx, &ClientConfig{
+			Backend:     BackendGeminiAPI,
+			APIKey:      "test-api-key",
+			HTTPOptions: customOptions,
+		})
+		if err != nil {
+			t.Fatalf("Expected no error, got %v", err)
+		}
+
+		// Verify HTTP options were correctly passed to the client config
+		if client.clientConfig.HTTPOptions.APIVersion != customOptions.APIVersion {
+			t.Errorf("Expected APIVersion %q, got %q", customOptions.APIVersion, client.clientConfig.HTTPOptions.APIVersion)
+		}
+		if client.clientConfig.HTTPOptions.BaseURL != customOptions.BaseURL {
+			t.Errorf("Expected BaseURL %q, got %q", customOptions.BaseURL, client.clientConfig.HTTPOptions.BaseURL)
+		}
+	})
+
+	// Test case: Gemini API with custom credentials and HTTP client
+	t.Run("GeminiAPI with custom credentials and HTTP client", func(t *testing.T) {
+		customHTTPClient := &http.Client{Timeout: 10 * time.Second}
+
+		client, err := NewClient(ctx, &ClientConfig{
+			Backend:     BackendGeminiAPI,
+			APIKey:      "test-api-key",
+			Credentials: mockCreds,
+			HTTPClient:  customHTTPClient,
+		})
+		if err != nil {
+			t.Fatalf("Expected no error, got %v", err)
+		}
+
+		// Verify custom HTTP client was used
+		if client.clientConfig.HTTPClient != customHTTPClient {
+			t.Errorf("Expected custom HTTP client to be used")
+		}
+	})
+}
+
 func TestClientConfigHTTPOptions(t *testing.T) {
 	tests := []struct {
 		name               string
diff --git a/example_test.go b/example_test.go
index 36ef64b8..12f3a0f4 100644
--- a/example_test.go
+++ b/example_test.go
@@ -21,7 +21,9 @@ import (
 	"io"
 	"log"
 	"net/http"
+	"time"
 
+	"cloud.google.com/go/auth"
 	"google.golang.org/genai"
 )
 
@@ -61,6 +63,47 @@ func ExampleNewClient_geminiapi() {
 	fmt.Println(client.ClientConfig().APIKey)
 }
 
+// This example shows how to create a new client for Gemini API with custom credentials and HTTP configuration.
+func ExampleNewClient_geminiapi_withCustomCredentials() {
+	ctx := context.Background()
+
+	// In a real application, you would create credentials from a service account
+	// For example, using auth.DetectDefault() or other credential methods
+	// Here we just demonstrate with a mock credentials object
+	creds := &auth.Credentials{}
+
+	// Create a custom HTTP client with specific timeouts
+	customHTTPClient := &http.Client{
+		Timeout: 30 * time.Second,
+	}
+
+	// Create client with custom credentials and HTTP configuration
+	client, err := genai.NewClient(ctx, &genai.ClientConfig{
+		APIKey:      apiKey,
+		Backend:     genai.BackendGeminiAPI,
+		Credentials: creds,
+		HTTPClient:  customHTTPClient,
+		HTTPOptions: genai.HTTPOptions{
+			APIVersion: "v1beta", // Specify API version if needed
+		},
+	})
+	if err != nil {
+		log.Fatalf("failed to create client: %v", err)
+	}
+
+	// The client now uses the custom credentials and HTTP configuration
+	// for all API calls
+	result, err := client.Models.GenerateContent(ctx,
+		"gemini-2.0-flash",
+		genai.Text("Tell me about cloud credentials?"),
+		nil,
+	)
+	if err != nil {
+		log.Fatal(err)
+	}
+	debugPrint(result)
+}
+
 // This example shows how to call the GenerateContent method with a simple text to Vertex AI.
 func ExampleModels_GenerateContent_text_vertexai() {
 	ctx := context.Background()