diff --git a/src/service.ts b/src/service.ts
index e2d5f312..c262039c 100644
--- a/src/service.ts
+++ b/src/service.ts
@@ -69,6 +69,7 @@ export interface ServiceOptions extends GoogleAuthOptions {
   token?: string;
   timeout?: number; // http.request.options.timeout
   userAgent?: string;
+  useAuthWithCustomEndpoint?: boolean;
 }
 
 export class Service {
diff --git a/src/util.ts b/src/util.ts
index 189fa553..88f756bf 100644
--- a/src/util.ts
+++ b/src/util.ts
@@ -138,6 +138,11 @@ export interface MakeAuthenticatedRequestFactoryConfig
    */
   customEndpoint?: boolean;
 
+  /**
+   * If true, will authenticate when using a custom endpoint. Default: false.
+   */
+  useAuthWithCustomEndpoint?: boolean;
+
   /**
    * Account email address, required for PEM/P12 usage.
    */
@@ -572,6 +577,7 @@ export class Util {
    * (default: true)
    * @param {object=} config.credentials - Credentials object.
    * @param {boolean=} config.customEndpoint - If true, just return the provided request options. Default: false.
+   * @param {boolean=} config.useAuthWithCustomEndpoint - If true, will authenticate when using a custom endpoint. Default: false.
    * @param {string=} config.email - Account email address, required for PEM/P12 usage.
    * @param {number=} config.maxRetries - Maximum number of automatic retries attempted before returning the error. (default: 3)
    * @param {string=} config.keyFile - Path to a .json, .pem, or .p12 keyfile.
@@ -697,7 +703,7 @@ export class Util {
             // auth client, it could be incorrect.
             new Promise(resolve => resolve(config.projectId))
           : authClient.getProjectId(),
-        reqConfig.customEndpoint
+        reqConfig.customEndpoint && reqConfig.useAuthWithCustomEndpoint !== true
           ? // Using a custom API override. Do not use `google-auth-library` for
             // authentication. (ex: connecting to a local Datastore server)
             new Promise(resolve => resolve(reqOpts))
diff --git a/test/util.ts b/test/util.ts
index 56cce4b9..0d16f944 100644
--- a/test/util.ts
+++ b/test/util.ts
@@ -835,6 +835,33 @@ describe('common/util', () => {
       });
     });
 
+    describe('customEndpoint (authentication attempted)', () => {
+      // eslint-disable-next-line @typescript-eslint/no-explicit-any
+      let makeAuthenticatedRequest: any;
+      const config = {customEndpoint: true, useAuthWithCustomEndpoint: true};
+
+      beforeEach(() => {
+        sandbox.stub(fakeGoogleAuth, 'GoogleAuth').returns(authClient);
+        makeAuthenticatedRequest = util.makeAuthenticatedRequestFactory(config);
+      });
+
+      it('should authenticate requests with a custom API', done => {
+        const reqOpts = {a: 'b', c: 'd'};
+
+        stub('makeRequest', rOpts => {
+          assert.deepStrictEqual(rOpts, reqOpts);
+          done();
+        });
+
+        authClient.authorizeRequest = async (opts: {}) => {
+          assert.strictEqual(opts, reqOpts);
+          done();
+        };
+
+        makeAuthenticatedRequest(reqOpts, assert.ifError);
+      });
+    });
+
     describe('needs authentication', () => {
       it('should pass correct args to authorizeRequest', done => {
         const fake = extend(true, authClient, {