diff --git a/etc/firebase-admin.api.md b/etc/firebase-admin.api.md
index b2f107e95f..9005e91a07 100644
--- a/etc/firebase-admin.api.md
+++ b/etc/firebase-admin.api.md
@@ -498,15 +498,18 @@ export interface GoogleOAuthAccessToken {
 export function initializeApp(options?: AppOptions, name?: string): app.App;
 
 // @public
-export function instanceId(app?: app.App): instanceId.InstanceId;
+export class InstanceId {
+    get app(): App;
+    deleteInstanceId(instanceId: string): Promise<void>;
+    }
+
+// @public
+export function instanceId(app?: App): InstanceId;
 
 // @public (undocumented)
 export namespace instanceId {
-    export interface InstanceId {
-        // (undocumented)
-        app: app.App;
-        deleteInstanceId(instanceId: string): Promise<void>;
-    }
+    // (undocumented)
+    export type InstanceId = InstanceId;
 }
 
 // @public
diff --git a/gulpfile.js b/gulpfile.js
index 1e1adcbba6..0adae29789 100644
--- a/gulpfile.js
+++ b/gulpfile.js
@@ -88,6 +88,7 @@ gulp.task('compile', function() {
     'lib/firebase-namespace-api.d.ts',
     'lib/core.d.ts',
     'lib/app/*.d.ts',
+    'lib/instance-id/*.d.ts',
     '!lib/utils/index.d.ts',
   ];
 
diff --git a/src/app/firebase-app.ts b/src/app/firebase-app.ts
index 80f0fb2088..bcd8819f23 100644
--- a/src/app/firebase-app.ts
+++ b/src/app/firebase-app.ts
@@ -31,7 +31,7 @@ import { database } from '../database/index';
 import { DatabaseService } from '../database/database-internal';
 import { Firestore } from '@google-cloud/firestore';
 import { FirestoreService } from '../firestore/firestore-internal';
-import { InstanceId } from '../instance-id/instance-id';
+import { InstanceId } from '../instance-id/index';
 import { ProjectManagement } from '../project-management/project-management';
 import { SecurityRules } from '../security-rules/security-rules';
 import { RemoteConfig } from '../remote-config/remote-config';
@@ -231,6 +231,8 @@ export class FirebaseAppInternals {
 
 /**
  * Global context object for a collection of services using a shared authentication state.
+ *
+ * @internal
  */
 export class FirebaseApp implements app.App {
   public INTERNAL: FirebaseAppInternals;
@@ -333,10 +335,8 @@ export class FirebaseApp implements app.App {
    * @return The InstanceId service instance of this app.
    */
   public instanceId(): InstanceId {
-    return this.ensureService_('iid', () => {
-      const iidService: typeof InstanceId = require('../instance-id/instance-id').InstanceId;
-      return new iidService(this);
-    });
+    const fn = require('../instance-id/index').instanceId;
+    return fn(this);
   }
 
   /**
@@ -410,6 +410,13 @@ export class FirebaseApp implements app.App {
     return deepCopy(this.options_);
   }
 
+  /**
+   * @internal
+   */
+  public getOrInitService<T>(name: string, init: (app: FirebaseApp) => T): T {
+    return this.ensureService_(name, () => init(this));
+  }
+
   /**
    * Deletes the FirebaseApp instance.
    *
diff --git a/src/instance-id/index.ts b/src/instance-id/index.ts
index 09cbfe4022..53b12bffcc 100644
--- a/src/instance-id/index.ts
+++ b/src/instance-id/index.ts
@@ -14,7 +14,11 @@
  * limitations under the License.
  */
 
-import { app } from '../firebase-namespace-api';
+import { FirebaseApp } from '../app/firebase-app';
+import { App, getApp } from '../app/index';
+import { InstanceId } from './instance-id';
+
+export { InstanceId };
 
 /**
  * Gets the {@link instanceId.InstanceId `InstanceId`} service for the
@@ -46,40 +50,18 @@ import { app } from '../firebase-namespace-api';
  *   no app is provided or the `InstanceId` service associated with the
  *   provided app.
  */
-export declare function instanceId(app?: app.App): instanceId.InstanceId;
+export function instanceId(app?: App): InstanceId {
+  if (typeof app === 'undefined') {
+    app = getApp();
+  }
+
+  const firebaseApp: FirebaseApp = app as FirebaseApp;
+  return firebaseApp.getOrInitService('instanceId', (app) => new InstanceId(app));
+}
+
+import { InstanceId as TInstanceId } from './instance-id';
 
 /* eslint-disable @typescript-eslint/no-namespace */
 export namespace instanceId {
-  /**
-   * Gets the {@link InstanceId `InstanceId`} service for the
-   * current app.
-   *
-   * @example
-   * ```javascript
-   * var instanceId = app.instanceId();
-   * // The above is shorthand for:
-   * // var instanceId = admin.instanceId(app);
-   * ```
-   *
-   * @return The `InstanceId` service for the
-   *   current app.
-   */
-  export interface InstanceId {
-    app: app.App;
-
-    /**
-     * Deletes the specified instance ID and the associated data from Firebase.
-     *
-     * Note that Google Analytics for Firebase uses its own form of Instance ID to
-     * keep track of analytics data. Therefore deleting a Firebase Instance ID does
-     * not delete Analytics data. See
-     * [Delete an Instance ID](/support/privacy/manage-iids#delete_an_instance_id)
-     * for more information.
-     *
-     * @param instanceId The instance ID to be deleted.
-     *
-     * @return A promise fulfilled when the instance ID is deleted.
-     */
-    deleteInstanceId(instanceId: string): Promise<void>;
-  }
+  export type InstanceId = TInstanceId;
 }
diff --git a/src/instance-id/instance-id-request-internal.ts b/src/instance-id/instance-id-request-internal.ts
index fbd3ba15ed..1ef9125744 100644
--- a/src/instance-id/instance-id-request-internal.ts
+++ b/src/instance-id/instance-id-request-internal.ts
@@ -15,6 +15,7 @@
  * limitations under the License.
  */
 
+import { App } from '../app/index';
 import { FirebaseApp } from '../app/firebase-app';
 import { FirebaseInstanceIdError, InstanceIdClientErrorCode } from '../utils/error';
 import {
@@ -54,12 +55,12 @@ export class FirebaseInstanceIdRequestHandler {
   private path: string;
 
   /**
-   * @param {FirebaseApp} app The app used to fetch access tokens to sign API requests.
+   * @param app The app used to fetch access tokens to sign API requests.
    *
    * @constructor
    */
-  constructor(private readonly app: FirebaseApp) {
-    this.httpClient = new AuthorizedHttpClient(app);
+  constructor(private readonly app: App) {
+    this.httpClient = new AuthorizedHttpClient(app as FirebaseApp);
   }
 
   public deleteInstanceId(instanceId: string): Promise<void> {
diff --git a/src/instance-id/instance-id.ts b/src/instance-id/instance-id.ts
index b92662bfd7..1af8bdb151 100644
--- a/src/instance-id/instance-id.ts
+++ b/src/instance-id/instance-id.ts
@@ -14,14 +14,11 @@
  * limitations under the License.
  */
 
-import { FirebaseApp } from '../app/firebase-app';
+import { App } from '../app/index';
 import { FirebaseInstanceIdError, InstanceIdClientErrorCode } from '../utils/error';
 import { FirebaseInstanceIdRequestHandler } from './instance-id-request-internal';
-import { instanceId } from './index';
 import * as validator from '../utils/validator';
 
-import InstanceIdInterface = instanceId.InstanceId;
-
 /**
  * Gets the {@link InstanceId `InstanceId`} service for the
  * current app.
@@ -36,20 +33,21 @@ import InstanceIdInterface = instanceId.InstanceId;
  * @return The `InstanceId` service for the
  *   current app.
  */
-export class InstanceId implements InstanceIdInterface {
+export class InstanceId {
 
-  private app_: FirebaseApp;
+  private app_: App;
   private requestHandler: FirebaseInstanceIdRequestHandler;
 
   /**
-   * @param {FirebaseApp} app The app for this InstanceId service.
+   * @param app The app for this InstanceId service.
    * @constructor
+   * @internal
    */
-  constructor(app: FirebaseApp) {
+  constructor(app: App) {
     if (!validator.isNonNullObject(app) || !('options' in app)) {
       throw new FirebaseInstanceIdError(
         InstanceIdClientErrorCode.INVALID_ARGUMENT,
-        'First argument passed to admin.instanceId() must be a valid Firebase app instance.',
+        'First argument passed to instanceId() must be a valid Firebase app instance.',
       );
     }
 
@@ -80,9 +78,9 @@ export class InstanceId implements InstanceIdInterface {
   /**
    * Returns the app associated with this InstanceId instance.
    *
-   * @return {FirebaseApp} The app associated with this InstanceId instance.
+   * @return The app associated with this InstanceId instance.
    */
-  get app(): FirebaseApp {
+  get app(): App {
     return this.app_;
   }
 }
diff --git a/src/utils/index.ts b/src/utils/index.ts
index b8cfa2faf0..834b8cb6cd 100644
--- a/src/utils/index.ts
+++ b/src/utils/index.ts
@@ -15,7 +15,7 @@
  * limitations under the License.
  */
 
-import { app as _app } from '../firebase-namespace-api';
+import { App } from '../app/index';
 import {
   ServiceAccountCredential, ComputeEngineCredential
 } from '../credential/credential-internal';
@@ -23,6 +23,7 @@ import * as validator from './validator';
 
 let sdkVersion: string;
 
+// TODO: Move to firebase-admin/app as an internal member.
 export function getSdkVersion(): string {
   if (!sdkVersion) {
     const { version } = require('../../package.json'); // eslint-disable-line @typescript-eslint/no-var-requires
@@ -76,7 +77,7 @@ export function addReadonlyGetter(obj: object, prop: string, value: any): void {
  *
  * @return A project ID string or null.
  */
-export function getExplicitProjectId(app: _app.App): string | null {
+export function getExplicitProjectId(app: App): string | null {
   const options = app.options;
   if (validator.isNonEmptyString(options.projectId)) {
     return options.projectId;
@@ -105,7 +106,7 @@ export function getExplicitProjectId(app: _app.App): string | null {
  *
  * @return A project ID string or null.
  */
-export function findProjectId(app: _app.App): Promise<string | null> {
+export function findProjectId(app: App): Promise<string | null> {
   const projectId = getExplicitProjectId(app);
   if (projectId) {
     return Promise.resolve(projectId);
diff --git a/test/unit/index.spec.ts b/test/unit/index.spec.ts
index 887ccb5642..a7321f8cb0 100644
--- a/test/unit/index.spec.ts
+++ b/test/unit/index.spec.ts
@@ -60,6 +60,7 @@ import './storage/storage.spec';
 import './firestore/firestore.spec';
 
 // InstanceId
+import './instance-id/index.spec';
 import './instance-id/instance-id.spec';
 import './instance-id/instance-id-request.spec';
 
diff --git a/test/unit/instance-id/index.spec.ts b/test/unit/instance-id/index.spec.ts
new file mode 100644
index 0000000000..125d0ca6fa
--- /dev/null
+++ b/test/unit/instance-id/index.spec.ts
@@ -0,0 +1,75 @@
+/*!
+ * @license
+ * Copyright 2021 Google Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+'use strict';
+
+import * as chai from 'chai';
+import * as sinonChai from 'sinon-chai';
+import * as chaiAsPromised from 'chai-as-promised';
+
+import * as mocks from '../../resources/mocks';
+import { App } from '../../../src/app/index';
+import { instanceId, InstanceId } from '../../../src/instance-id/index';
+
+chai.should();
+chai.use(sinonChai);
+chai.use(chaiAsPromised);
+
+const expect = chai.expect;
+
+describe('InstanceId', () => {
+  let mockApp: App;
+  let mockCredentialApp: App;
+
+  const noProjectIdError = 'Failed to determine project ID for InstanceId. Initialize the SDK '
+  + 'with service account credentials or set project ID as an app option. Alternatively set the '
+  + 'GOOGLE_CLOUD_PROJECT environment variable.';
+
+  beforeEach(() => {
+    mockApp = mocks.app();
+    mockCredentialApp = mocks.mockCredentialApp();
+  });
+
+  describe('instanceId()', () => {
+    it('should throw when default app is not available', () => {
+      expect(() => {
+        return instanceId();
+      }).to.throw('The default Firebase app does not exist.');
+    });
+
+    it('should reject given an invalid credential without project ID', () => {
+      // Project ID not set in the environment.
+      delete process.env.GOOGLE_CLOUD_PROJECT;
+      delete process.env.GCLOUD_PROJECT;
+      const iid = instanceId(mockCredentialApp);
+      return iid.deleteInstanceId('iid')
+        .should.eventually.rejectedWith(noProjectIdError);
+    });
+
+    it('should not throw given a valid app', () => {
+      expect(() => {
+        return instanceId(mockApp);
+      }).not.to.throw();
+    });
+
+    it('should return the same instance for a given app instance', () => {
+      const iid1: InstanceId = instanceId(mockApp);
+      const iid2: InstanceId = instanceId(mockApp);
+      expect(iid1).to.equal(iid2);
+    });
+  });
+});
diff --git a/test/unit/instance-id/instance-id.spec.ts b/test/unit/instance-id/instance-id.spec.ts
index a2b2650df8..90bbacdb8d 100644
--- a/test/unit/instance-id/instance-id.spec.ts
+++ b/test/unit/instance-id/instance-id.spec.ts
@@ -83,7 +83,7 @@ describe('InstanceId', () => {
         expect(() => {
           const iidAny: any = InstanceId;
           return new iidAny(invalidApp);
-        }).to.throw('First argument passed to admin.instanceId() must be a valid Firebase app instance.');
+        }).to.throw('First argument passed to instanceId() must be a valid Firebase app instance.');
       });
     });
 
@@ -91,7 +91,7 @@ describe('InstanceId', () => {
       expect(() => {
         const iidAny: any = InstanceId;
         return new iidAny();
-      }).to.throw('First argument passed to admin.instanceId() must be a valid Firebase app instance.');
+      }).to.throw('First argument passed to instanceId() must be a valid Firebase app instance.');
     });
 
     it('should reject given an invalid credential without project ID', () => {