diff --git a/changelog/fixed/relay-racing-raccoon.md b/changelog/fixed/relay-racing-raccoon.md
new file mode 100644
index 000000000..d2d84df6b
--- /dev/null
+++ b/changelog/fixed/relay-racing-raccoon.md
@@ -0,0 +1 @@
+Setting `Worker.client` no longer raises "New client is not on the same runtime as the existing client" when both clients connected without a `runtime`, since both use the default runtime. The check now compares the runtime each client actually connected with.
diff --git a/temporalio/service.py b/temporalio/service.py
index 926dfbd46..5c390f92a 100644
--- a/temporalio/service.py
+++ b/temporalio/service.py
@@ -418,6 +418,8 @@ def __init__(self, config: ConnectConfig) -> None:
         self._bridge_config = config._to_bridge_config()
         self._bridge_client: temporalio.bridge.client.Client | None = None
         self._bridge_client_connect_lock = asyncio.Lock()
+        # Runtime we connected with, so the default one if none was set
+        self._runtime: temporalio.runtime.Runtime | None = None
 
     async def _connected_client(self) -> temporalio.bridge.client.Client:
         # Fast path avoids touching the lock once connected. This keeps the
@@ -433,6 +435,7 @@ async def _connected_client(self) -> temporalio.bridge.client.Client:
                     runtime._core_runtime,
                     self._bridge_config,
                 )
+                self._runtime = runtime
             return self._bridge_client
 
     @property
diff --git a/temporalio/worker/_worker.py b/temporalio/worker/_worker.py
index 39faf615a..3c6effd80 100644
--- a/temporalio/worker/_worker.py
+++ b/temporalio/worker/_worker.py
@@ -491,7 +491,9 @@ def _init_from_config(self, client: temporalio.client.Client, config: WorkerConf
 
         self._activity_worker: _ActivityWorker | None = None
         self._runtime = (
-            bridge_client.config.runtime or temporalio.runtime.Runtime.default()
+            bridge_client._runtime
+            or bridge_client.config.runtime
+            or temporalio.runtime.Runtime.default()
         )
         activities = config.get("activities")
         if activities:
@@ -743,7 +745,7 @@ def client(self, value: temporalio.client.Client) -> None:
         must be using the same runtime as the current client.
         """
         bridge_client = _extract_bridge_client_for_worker(value)
-        if self._runtime is not bridge_client.config.runtime:
+        if self._runtime is not bridge_client._runtime:
             raise ValueError(
                 "New client is not on the same runtime as the existing client"
             )
diff --git a/tests/worker/test_workflow.py b/tests/worker/test_workflow.py
index 80ac3d27b..01a2fcea8 100644
--- a/tests/worker/test_workflow.py
+++ b/tests/worker/test_workflow.py
@@ -6669,6 +6669,33 @@ async def test_workflow_replace_worker_client_diff_runtimes_fail(
             worker.client = other_client
 
 
+async def test_workflow_replace_worker_client_implicit_default_runtime(
+    env: WorkflowEnvironment,
+):
+    # Neither client sets a runtime, so both are on the default one
+    worker_client = await env.connect_client(runtime=None)
+    new_client = await env.connect_client(runtime=None)
+    async with new_worker(worker_client, HelloWorkflow) as worker:
+        worker.client = new_client
+        assert worker.client is new_client
+
+
+async def test_workflow_replace_worker_client_implicit_default_runtime_mismatch_fail(
+    env: WorkflowEnvironment,
+):
+    # No runtime means the default one, which isn't the worker's
+    worker_client = await env.connect_client(
+        runtime=Runtime(telemetry=TelemetryConfig())
+    )
+    default_runtime_client = await env.connect_client(runtime=None)
+    async with new_worker(worker_client, HelloWorkflow) as worker:
+        with pytest.raises(
+            ValueError,
+            match="New client is not on the same runtime as the existing client",
+        ):
+            worker.client = default_runtime_client
+
+
 @activity.defn(dynamic=True)
 async def return_name_activity(_args: Sequence[RawValue]) -> str:
     return activity.info().activity_type