diff --git a/CHANGELOG.md b/CHANGELOG.md
index 5474179818..ba9acc9c8c 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -41,6 +41,9 @@ to docs, or any other relevant information.
   specific to the Temporal CLI dev server implementation and may no longer be valid if that implementation changes.
 
 ### Fixed
+- `ActivityClient` now sends the `temporal-namespace` gRPC header like `WorkflowClient` and `ScheduleClient` do,
+  so standalone activity calls are no longer rejected with `PERMISSION_DENIED` by namespace-scoped auth (e.g.
+  Temporal Cloud API keys).
 - Kotlin `Async` method references compiled with Kotlin 2.4 now retain their inline behavior,
   preventing `NonDeterministicException` when replaying workflows started with Kotlin 2.2. For
   workflows already started with Kotlin 2.4 and SDK 1.40 or earlier, set the
diff --git a/temporal-sdk/src/main/java/io/temporal/client/ActivityClientImpl.java b/temporal-sdk/src/main/java/io/temporal/client/ActivityClientImpl.java
index 77bf2bd794..52d8b31831 100644
--- a/temporal-sdk/src/main/java/io/temporal/client/ActivityClientImpl.java
+++ b/temporal-sdk/src/main/java/io/temporal/client/ActivityClientImpl.java
@@ -8,6 +8,7 @@
 import io.temporal.common.interceptors.Header;
 import io.temporal.internal.client.ActivityClientInternal;
 import io.temporal.internal.client.ActivityHandleImpl;
+import io.temporal.internal.client.NamespaceInjectWorkflowServiceStubs;
 import io.temporal.internal.client.RootActivityClientInvoker;
 import io.temporal.internal.client.external.GenericWorkflowClientImpl;
 import io.temporal.internal.client.external.ManualActivityCompletionClientFactory;
@@ -39,13 +40,19 @@ class ActivityClientImpl implements ActivityClient, ActivityClientInternal {
   ActivityClientImpl(WorkflowServiceStubs stubs, ActivityClientOptions options) {
     this.stubs = stubs;
     this.options = options;
+    WorkflowServiceStubs namespacedStubs =
+        new NamespaceInjectWorkflowServiceStubs(stubs, options.getNamespace());
     this.metricsScope =
         stubs.getOptions().getMetricsScope().tagged(MetricsTag.defaultTags(options.getNamespace()));
-    GenericWorkflowClientImpl genericClient = new GenericWorkflowClientImpl(stubs, metricsScope);
+    GenericWorkflowClientImpl genericClient =
+        new GenericWorkflowClientImpl(namespacedStubs, metricsScope);
     this.invoker = initializeClientInvoker(genericClient, options);
     this.manualActivityCompletionClientFactory =
         ManualActivityCompletionClientFactory.newFactory(
-            stubs, options.getNamespace(), options.getIdentity(), options.getDataConverter());
+            namespacedStubs,
+            options.getNamespace(),
+            options.getIdentity(),
+            options.getDataConverter());
   }
 
   private static ActivityClientCallsInterceptor initializeClientInvoker(
diff --git a/temporal-sdk/src/test/java/io/temporal/authorization/AuthorizationTokenTest.java b/temporal-sdk/src/test/java/io/temporal/authorization/AuthorizationTokenTest.java
index b69327eae6..811fdfee36 100644
--- a/temporal-sdk/src/test/java/io/temporal/authorization/AuthorizationTokenTest.java
+++ b/temporal-sdk/src/test/java/io/temporal/authorization/AuthorizationTokenTest.java
@@ -3,6 +3,8 @@
 import static org.junit.Assert.*;
 
 import io.grpc.*;
+import io.temporal.client.ActivityClient;
+import io.temporal.client.StartActivityOptions;
 import io.temporal.client.WorkflowClient;
 import io.temporal.client.WorkflowOptions;
 import io.temporal.serviceclient.WorkflowServiceStubsOptions;
@@ -18,6 +20,7 @@
 import java.util.Arrays;
 import java.util.Collections;
 import java.util.List;
+import java.util.stream.Collectors;
 import org.junit.After;
 import org.junit.Before;
 import org.junit.Rule;
@@ -125,6 +128,40 @@ public void allRequestsShouldHaveAnAuthToken() {
     }
   }
 
+  @Test
+  public void activityClientRequestsShouldHaveANamespace() {
+    testEnvironment.start();
+    ActivityClient client = testEnvironment.getActivityClient();
+    StartActivityOptions options =
+        StartActivityOptions.newBuilder()
+            .setId("activity-id")
+            .setTaskQueue(TASK_QUEUE)
+            .setStartToCloseTimeout(Duration.ofMinutes(1))
+            .build();
+
+    // Only the outgoing request headers matter, not whether the server supports the call.
+    try {
+      client.start("SomeActivity", options);
+    } catch (StatusRuntimeException ignored) {
+    }
+
+    List<GrpcRequest> startRequests =
+        loggedRequests.stream()
+            .filter(request -> request.methodName.equals("StartActivityExecution"))
+            .collect(Collectors.toList());
+    assertFalse("StartActivityExecution should have been called", startRequests.isEmpty());
+    for (GrpcRequest grpcRequest : startRequests) {
+      assertEquals(
+          "StartActivityExecution should have an auth token",
+          AUTH_TOKEN,
+          grpcRequest.authTokenValue);
+      assertEquals(
+          "StartActivityExecution should have a namespace",
+          testEnvironment.getNamespace(),
+          grpcRequest.namespace);
+    }
+  }
+
   public static class EmptyWorkflowImpl implements TestWorkflows.TestWorkflow1 {
 
     @Override