diff --git a/src/clients/apim-client.ts b/src/clients/apim-client.ts
index 5d9eaf3..99cfbf3 100644
--- a/src/clients/apim-client.ts
+++ b/src/clients/apim-client.ts
@@ -207,9 +207,18 @@ export class ApimClient implements IApimClient {
 
         return response;
       } catch (error) {
-        // Do not retry client errors (4xx) — they are deterministic, not transient.
-        // 429 rate-limiting is already handled above and never reaches here.
-        if (error instanceof HttpError && error.status >= 400 && error.status < 500) {
+        // APIM reports operations blocked by an API's in-progress async operation
+        // as a transient 409. Other client errors are deterministic.
+        const isPessimisticConcurrencyConflict =
+          error instanceof HttpError &&
+          error.status === 409 &&
+          error.code === 'PessimisticConcurrencyConflict';
+        if (
+          error instanceof HttpError &&
+          error.status >= 400 &&
+          error.status < 500 &&
+          !isPessimisticConcurrencyConflict
+        ) {
           throw error;
         }
         if (attempt >= ApimClient.MAX_RETRIES) {
diff --git a/tests/unit/clients/apim-client.test.ts b/tests/unit/clients/apim-client.test.ts
index 9864fac..5590b3f 100644
--- a/tests/unit/clients/apim-client.test.ts
+++ b/tests/unit/clients/apim-client.test.ts
@@ -887,6 +887,72 @@ describe('ApimClient HTTP 429 rate limiting', () => {
   });
 });
 
+describe('ApimClient HTTP 409 conflict handling', () => {
+  let client: ApimClient;
+  let fetchSpy: ReturnType<typeof vi.fn>;
+
+  beforeEach(() => {
+    client = new ApimClient();
+    fetchSpy = vi.fn();
+    vi.stubGlobal('fetch', fetchSpy);
+
+    /* eslint-disable @typescript-eslint/no-explicit-any */
+    vi.spyOn(client as any, 'getToken').mockResolvedValue('fake-token');
+    vi.spyOn(client as any, 'delay').mockResolvedValue(undefined);
+    /* eslint-enable @typescript-eslint/no-explicit-any */
+  });
+
+  afterEach(() => {
+    vi.restoreAllMocks();
+    vi.unstubAllGlobals();
+  });
+
+  it('should retry PessimisticConcurrencyConflict responses on PUT operations', async () => {
+    fetchSpy
+      .mockResolvedValueOnce(
+        makeResponse(409, {
+          error: {
+            code: 'PessimisticConcurrencyConflict',
+            message: 'Operation on the API is in progress',
+          },
+        })
+      )
+      .mockResolvedValueOnce(makeResponse(200, { name: 'my-api' }));
+
+    const descriptor = {
+      type: ResourceType.Api,
+      nameParts: ['my-api'],
+    };
+
+    await expect(client.putResource(testContext, descriptor, {}))
+      .resolves.toEqual({ name: 'my-api' });
+    expect(fetchSpy).toHaveBeenCalledTimes(2);
+  });
+
+  it('should not retry unrelated HTTP 409 responses', async () => {
+    fetchSpy.mockResolvedValueOnce(
+      makeResponse(409, {
+        error: {
+          code: 'AnotherConflict',
+          message: 'A non-transient conflict',
+        },
+      })
+    );
+
+    const descriptor = {
+      type: ResourceType.Api,
+      nameParts: ['my-api'],
+    };
+
+    await expect(client.putResource(testContext, descriptor, {}))
+      .rejects.toMatchObject({
+        status: 409,
+        code: 'AnotherConflict',
+      });
+    expect(fetchSpy).toHaveBeenCalledTimes(1);
+  });
+});
+
 describe('ApimClient.getApiSpecification', () => {
   let client: ApimClient;
   let fetchSpy: ReturnType<typeof vi.fn>;