diff --git a/packages/ember-simple-auth/index.d.ts b/packages/ember-simple-auth/index.d.ts
new file mode 100644
index 000000000..23b79745c
--- /dev/null
+++ b/packages/ember-simple-auth/index.d.ts
@@ -0,0 +1,140 @@
+interface Callback {
+  (): void;
+}
+type Data = Record<string, unknown>;
+interface SessionData<T> {
+  authenticated: T;
+}
+
+declare module 'ember-simple-auth/services/session' {
+  import Evented from '@ember/object/evented';
+  import Service from '@ember/service';
+  import type BaseSessionStore from 'ember-simple-auth/session-stores/base';
+  import type Transition from '@ember/routing/-private/transition';
+
+  export default class SessionService<T> extends Service.extend(Evented) {
+    isAuthenticated: boolean;
+    data: SessionData<T>;
+    store: BaseSessionStore;
+    attemptedTransition: Transition | null;
+
+    authenticate(authenticator: string, ...args: unknown[]): Promise<void>;
+    invalidate(...args: unknown[]): Promise<unknown>;
+    requireAuthentication(transition: Transition, routeOrCallback: string | Callback): boolean;
+    prohibitAuthentication(routeOrCallback: string | Callback): boolean;
+    handleAuthentication(routeAfterAuthentication: string): void;
+    handleInvalidation(routeAfterInvalidation: string): void;
+    setup(): Promise<void>;
+  }
+}
+
+declare module 'ember-simple-auth/authenticators/base' {
+  import EmberObject from '@ember/object';
+  import Evented from '@ember/object/evented';
+
+  export default class extends EmberObject.extend(Evented) {
+    restore(data: Data): Promise<unknown>;
+    authenticate(...args: unknown[]): Promise<unknown>;
+    invalidate(data: Data, ...args: unknown[]): Promise<unknown>;
+  }
+}
+
+declare module 'ember-simple-auth/session-stores/base' {
+  import EmberObject from '@ember/object';
+  import Evented from '@ember/object/evented';
+
+  export default class extends EmberObject.extend(Evented) {
+    persist(data: SessionData<Data>): Promise<unknown>;
+    restore(): Promise<SessionData<Data>>;
+    clear(): Promise<void>;
+  }
+}
+
+declare module 'ember-simple-auth/session-stores/local-storage' {
+  import BaseSessionStore from 'ember-simple-auth/session-stores/base';
+
+  export default class extends BaseSessionStore {
+    key: string;
+  }
+}
+
+declare module 'ember-simple-auth/session-stores/cookie' {
+  import BaseSessionStore from 'ember-simple-auth/session-stores/base';
+
+  export default class extends BaseSessionStore {
+    cookieDomain: string;
+    cookieExpirationTime: null | number;
+    cookieName: string;
+    cookiePath: string;
+    sameSite: null | 'Strict' | 'Lax';
+  }
+}
+
+declare module 'ember-simple-auth/session-stores/adaptive' {
+  import CookieSessionStore from 'ember-simple-auth/session-stores/base';
+
+  export default class extends CookieSessionStore {
+    localStorageKey: string;
+  }
+}
+
+declare module 'ember-simple-auth/session-stores/ephemeral' {
+  import BaseSessionStore from 'ember-simple-auth/session-stores/base';
+
+  export default class extends BaseSessionStore { }
+}
+
+declare module 'ember-simple-auth/session-stores/session-storage' {
+  import BaseSessionStore from 'ember-simple-auth/session-stores/base';
+
+  export default class extends BaseSessionStore {
+    key: string;
+  }
+}
+
+declare module 'ember-simple-auth/authenticators/test' {
+  import BaseAuthenticator from 'ember-simple-auth/authenticators/base';
+
+  export default class extends BaseAuthenticator { }
+}
+
+declare module 'ember-simple-auth/authenticators/devise' {
+  import BaseAuthenticator from 'ember-simple-auth/authenticators/base';
+
+  export default class extends BaseAuthenticator {
+    serverTokenEndpoint: string;
+    resourceName: string;
+    tokenAttributeName: string;
+    identificationAttributeName: string;
+  }
+}
+
+declare module 'ember-simple-auth/authenticators/oauth2-implicit-grant' {
+  import BaseAuthenticator from 'ember-simple-auth/authenticators/base';
+
+  export default class extends BaseAuthenticator { }
+}
+
+declare module 'ember-simple-auth/authenticators/oauth2-password-grant' {
+  import BaseAuthenticator from 'ember-simple-auth/authenticators/base';
+
+  export default class extends BaseAuthenticator {
+    clientId: string | null;
+    serverTokenEndpoint: string;
+    serverTokenRevocationEndpoint: string | null;
+    refreshAccessTokens: boolean;
+  }
+}
+
+declare module 'ember-simple-auth/authenticators/torii' {
+  import BaseAuthenticator from 'ember-simple-auth/authenticators/base';
+
+  export default class extends BaseAuthenticator { }
+}
+
+declare module 'ember-simple-auth/test-support' {
+  import SimpleAuthSessionService from 'ember-simple-auth/services/session';
+  export function currentSession(): SimpleAuthSessionService<Data>;
+  export function authenticateSession(sessionData: Data): Promise<void>;
+  export function invalidateSession(): Promise<void>;
+}
diff --git a/packages/ember-simple-auth/package.json b/packages/ember-simple-auth/package.json
index b83ac9beb..e0ba8350f 100644
--- a/packages/ember-simple-auth/package.json
+++ b/packages/ember-simple-auth/package.json
@@ -12,6 +12,7 @@
     "doc": "doc",
     "test": "tests"
   },
+  "types": "./index.d.ts",
   "scripts": {
     "build": "ember build",
     "lint": "npm-run-all --aggregate-output --continue-on-error --parallel \"lint:!(fix)\"",
@@ -36,6 +37,7 @@
     "@embroider/test-setup": "^2.0.0",
     "@glimmer/component": "^1.0.0",
     "@glimmer/tracking": "^1.0.0",
+    "@types/ember": "4.0.3",
     "babel-eslint": "^10.1.0",
     "broccoli-asset-rev": "^3.0.0",
     "ember-auto-import": "^2.4.2",