diff --git a/packages/ai/README.md b/packages/ai/README.md
index 32071faa..c339b234 100644
--- a/packages/ai/README.md
+++ b/packages/ai/README.md
@@ -37,7 +37,7 @@ Unified LLM API with automatic model discovery, provider configuration, token an
   - [Environment Variables](#environment-variables-nodejs-only)
   - [Checking Environment Variables](#checking-environment-variables)
 - [OAuth Providers](#oauth-providers)
-  - [Vertex AI (ADC)](#vertex-ai-adc)
+  - [Vertex AI](#vertex-ai)
   - [CLI Login](#cli-login)
   - [Programmatic OAuth](#programmatic-oauth)
   - [Login Flow Example](#login-flow-example)
@@ -907,7 +907,7 @@ In Node.js environments, you can set environment variables to avoid passing API
 | Azure OpenAI | `AZURE_OPENAI_API_KEY` + `AZURE_OPENAI_BASE_URL` or `AZURE_OPENAI_RESOURCE_NAME` (optional `AZURE_OPENAI_API_VERSION`, `AZURE_OPENAI_DEPLOYMENT_NAME_MAP` like `model=deployment,model2=deployment2`) |
 | Anthropic | `ANTHROPIC_API_KEY` or `ANTHROPIC_OAUTH_TOKEN` |
 | Google | `GEMINI_API_KEY` |
-| Vertex AI | `GOOGLE_CLOUD_PROJECT` (or `GCLOUD_PROJECT`) + `GOOGLE_CLOUD_LOCATION` + ADC |
+| Vertex AI | `GOOGLE_CLOUD_API_KEY` or `GOOGLE_CLOUD_PROJECT` (or `GCLOUD_PROJECT`) + `GOOGLE_CLOUD_LOCATION` + ADC |
 | Mistral | `MISTRAL_API_KEY` |
 | Groq | `GROQ_API_KEY` |
 | Cerebras | `CEREBRAS_API_KEY` |
@@ -975,14 +975,15 @@ Several providers require OAuth authentication instead of static API keys:
 
 For paid Cloud Code Assist subscriptions, set `GOOGLE_CLOUD_PROJECT` or `GOOGLE_CLOUD_PROJECT_ID` to your project ID.
 
-### Vertex AI (ADC)
+### Vertex AI
 
-Vertex AI models use Application Default Credentials (ADC):
+Vertex AI models support either a Google Cloud API key or Application Default Credentials (ADC):
 
-- **Local development**: Run `gcloud auth application-default login`
-- **CI/Production**: Set `GOOGLE_APPLICATION_CREDENTIALS` to point to a service account JSON key file
+- **API key**: Set `GOOGLE_CLOUD_API_KEY` or pass `apiKey` in the call options.
+- **Local development (ADC)**: Run `gcloud auth application-default login`
+- **CI/Production (ADC)**: Set `GOOGLE_APPLICATION_CREDENTIALS` to point to a service account JSON key file
 
-Also set `GOOGLE_CLOUD_PROJECT` (or `GCLOUD_PROJECT`) and `GOOGLE_CLOUD_LOCATION`. You can also pass `project`/`location` in the call options.
+When using ADC, also set `GOOGLE_CLOUD_PROJECT` (or `GCLOUD_PROJECT`) and `GOOGLE_CLOUD_LOCATION`. You can also pass `project`/`location` in the call options. When using `GOOGLE_CLOUD_API_KEY`, `project` and `location` are not required.
 
 Example:
 
@@ -1003,6 +1004,8 @@ import { getModel, complete } from '@mariozechner/pi-ai';
   const model = getModel('google-vertex', 'gemini-2.5-flash');
   const response = await complete(model, {
     messages: [{ role: 'user', content: 'Hello from Vertex AI' }]
+  }, {
+    apiKey: process.env.GOOGLE_CLOUD_API_KEY,
   });
 
   for (const block of response.content) {
diff --git a/packages/ai/src/env-api-keys.ts b/packages/ai/src/env-api-keys.ts
index 749e074f..95e91417 100644
--- a/packages/ai/src/env-api-keys.ts
+++ b/packages/ai/src/env-api-keys.ts
@@ -73,9 +73,13 @@ export function getEnvApiKey(provider: any): string | undefined {
 		return process.env.ANTHROPIC_OAUTH_TOKEN || process.env.ANTHROPIC_API_KEY;
 	}
 
-	// Vertex AI uses Application Default Credentials, not API keys.
-	// Auth is configured via `gcloud auth application-default login`.
+	// Vertex AI supports either an explicit API key or Application Default Credentials
+	// Auth is configured via `gcloud auth application-default login`
 	if (provider === "google-vertex") {
+		if (process.env.GOOGLE_CLOUD_API_KEY) {
+			return process.env.GOOGLE_CLOUD_API_KEY;
+		}
+
 		const hasCredentials = hasVertexAdcCredentials();
 		const hasProject = !!(process.env.GOOGLE_CLOUD_PROJECT || process.env.GCLOUD_PROJECT);
 		const hasLocation = !!process.env.GOOGLE_CLOUD_LOCATION;
diff --git a/packages/ai/src/providers/google-vertex.ts b/packages/ai/src/providers/google-vertex.ts
index 7bdaf5eb..3f04b33f 100644
--- a/packages/ai/src/providers/google-vertex.ts
+++ b/packages/ai/src/providers/google-vertex.ts
@@ -84,9 +84,11 @@ export const streamGoogleVertex: StreamFunction<"google-vertex", GoogleVertexOpt
 		};
 
 		try {
-			const project = resolveProject(options);
-			const location = resolveLocation(options);
-			const client = createClient(model, project, location, options?.headers);
+			const apiKey = resolveApiKey(options);
+			// Create the client using either a Vertex API key, if provided, or ADC with project and location
+			const client = apiKey
+				? createClientWithApiKey(model, apiKey, options?.headers)
+				: createClient(model, resolveProject(options), resolveLocation(options), options?.headers);
 			let params = buildParams(model, context, options);
 			const nextParams = await options?.onPayload?.(params, model);
 			if (nextParams !== undefined) {
@@ -341,6 +343,31 @@ function createClient(
 	});
 }
 
+function createClientWithApiKey(
+	model: Model<"google-vertex">,
+	apiKey: string,
+	optionsHeaders?: Record<string, string>,
+): GoogleGenAI {
+	const httpOptions: { headers?: Record<string, string> } = {};
+
+	if (model.headers || optionsHeaders) {
+		httpOptions.headers = { ...model.headers, ...optionsHeaders };
+	}
+
+	const hasHttpOptions = Object.values(httpOptions).some(Boolean);
+
+	return new GoogleGenAI({
+		vertexai: true,
+		apiKey,
+		apiVersion: API_VERSION,
+		httpOptions: hasHttpOptions ? httpOptions : undefined,
+	});
+}
+
+function resolveApiKey(options?: GoogleVertexOptions): string | undefined {
+	return options?.apiKey || process.env.GOOGLE_CLOUD_API_KEY;
+}
+
 function resolveProject(options?: GoogleVertexOptions): string {
 	const project = options?.project || process.env.GOOGLE_CLOUD_PROJECT || process.env.GCLOUD_PROJECT;
 	if (!project) {
diff --git a/packages/ai/test/stream.test.ts b/packages/ai/test/stream.test.ts
index 9b49f697..f4abb90a 100644
--- a/packages/ai/test/stream.test.ts
+++ b/packages/ai/test/stream.test.ts
@@ -380,6 +380,7 @@ describe("Generate E2E Tests", () => {
 	describe("Google Vertex Provider (gemini-3-flash-preview)", () => {
 		const vertexProject = process.env.GOOGLE_CLOUD_PROJECT || process.env.GCLOUD_PROJECT;
 		const vertexLocation = process.env.GOOGLE_CLOUD_LOCATION;
+		const vertexApiKey = process.env.GOOGLE_CLOUD_API_KEY;
 		const isVertexConfigured = Boolean(vertexProject && vertexLocation);
 		const vertexOptions = { project: vertexProject, location: vertexLocation } as const;
 		const llm = getModel("google-vertex", "gemini-3-flash-preview");
@@ -388,6 +389,10 @@ describe("Generate E2E Tests", () => {
 			await basicTextGeneration(llm, vertexOptions);
 		});
 
+		it.skipIf(!vertexApiKey)("should complete basic text generation with Vertex API key", { retry: 3 }, async () => {
+			await basicTextGeneration(llm, { apiKey: vertexApiKey! });
+		});
+
 		it.skipIf(!isVertexConfigured)("should handle tool calling", { retry: 3 }, async () => {
 			await handleToolCall(llm, vertexOptions);
 		});