diff --git a/.changeset/tidy-tokens-activate.md b/.changeset/tidy-tokens-activate.md
new file mode 100644
index 00000000000..27fb9354da7
--- /dev/null
+++ b/.changeset/tidy-tokens-activate.md
@@ -0,0 +1,5 @@
+---
+'@clerk/backend': minor
+---
+
+Add an optional `orgId` parameter to `createSignInToken()` for activating an Organization when the token is redeemed.
diff --git a/packages/backend/src/api/__tests__/SignInTokenApi.test.ts b/packages/backend/src/api/__tests__/SignInTokenApi.test.ts
new file mode 100644
index 00000000000..6d144fecbc9
--- /dev/null
+++ b/packages/backend/src/api/__tests__/SignInTokenApi.test.ts
@@ -0,0 +1,46 @@
+import { http, HttpResponse } from 'msw';
+import { describe, expect, it } from 'vitest';
+
+import { server, validateHeaders } from '../../mock-server';
+import { createBackendApiClient } from '../factory';
+
+describe('SignInTokenAPI', () => {
+  const apiClient = createBackendApiClient({
+    apiUrl: 'https://api.clerk.test',
+    secretKey: 'deadbeef',
+  });
+
+  it('sends the organization ID as org_id', async () => {
+    server.use(
+      http.post(
+        'https://api.clerk.test/v1/sign_in_tokens',
+        validateHeaders(async ({ request }) => {
+          const body = await request.json();
+          expect(body).toEqual({
+            user_id: 'user_123',
+            org_id: 'org_123',
+            expires_in_seconds: 3600,
+          });
+
+          return HttpResponse.json({
+            id: 'sign_in_token_123',
+            user_id: 'user_123',
+            token: 'token_123',
+            status: 'pending',
+            url: 'https://accounts.example.com/tickets/token_123',
+            created_at: 1,
+            updated_at: 1,
+          });
+        }),
+      ),
+    );
+
+    const response = await apiClient.signInTokens.createSignInToken({
+      userId: 'user_123',
+      orgId: 'org_123',
+      expiresInSeconds: 3600,
+    });
+
+    expect(response.id).toBe('sign_in_token_123');
+  });
+});
diff --git a/packages/backend/src/api/endpoints/SignInTokenApi.ts b/packages/backend/src/api/endpoints/SignInTokenApi.ts
index 0635e97e635..45cb23efb32 100644
--- a/packages/backend/src/api/endpoints/SignInTokenApi.ts
+++ b/packages/backend/src/api/endpoints/SignInTokenApi.ts
@@ -6,6 +6,8 @@ import { AbstractAPI } from './AbstractApi';
 export type CreateSignInTokensParams = {
   /** The ID of the user to create the sign-in token for. */
   userId: string;
+  /** The ID of the organization to activate when the user signs in. Organizations must be enabled for the instance, and the user must be a member of the organization. */
+  orgId?: string;
   /** The number of seconds until the sign-in token expires. By default, the duration is `2592000` (30 days). */
   expiresInSeconds: number;
 };