diff --git a/temporalio/client/_client.py b/temporalio/client/_client.py
index 1d8b8e4f2..438043f3d 100644
--- a/temporalio/client/_client.py
+++ b/temporalio/client/_client.py
@@ -1216,6 +1216,8 @@ def on_start_error(
         input = StartWorkflowUpdateWithStartInput(
             start_workflow_input=start_workflow_operation._start_workflow_input,
             update_workflow_input=update_input,
+            rpc_metadata=rpc_metadata,
+            rpc_timeout=rpc_timeout,
             _on_start=on_start,
             _on_start_error=on_start_error,
         )
diff --git a/temporalio/client/_impl.py b/temporalio/client/_impl.py
index e62f0b4a2..1481c8327 100644
--- a/temporalio/client/_impl.py
+++ b/temporalio/client/_impl.py
@@ -852,7 +852,11 @@ def on_start(
 
         try:
             return await self._start_workflow_update_with_start(
-                input.start_workflow_input, input.update_workflow_input, on_start
+                input.start_workflow_input,
+                input.update_workflow_input,
+                input.rpc_metadata,
+                input.rpc_timeout,
+                on_start,
             )
         except asyncio.CancelledError as _err:
             err = _err
@@ -914,6 +918,8 @@ async def _start_workflow_update_with_start(
         self,
         start_input: UpdateWithStartStartWorkflowInput,
         update_input: UpdateWithStartUpdateWorkflowInput,
+        rpc_metadata: Mapping[str, str | bytes],
+        rpc_timeout: timedelta | None,
         on_start: Callable[
             [temporalio.api.workflowservice.v1.StartWorkflowExecutionResponse], None
         ],
@@ -941,7 +947,12 @@ async def _start_workflow_update_with_start(
         # Repeatedly try to invoke ExecuteMultiOperation until the update is durable
         while True:
             multiop_response = (
-                await self._client.workflow_service.execute_multi_operation(multiop_req)
+                await self._client.workflow_service.execute_multi_operation(
+                    multiop_req,
+                    retry=True,
+                    metadata=rpc_metadata,
+                    timeout=rpc_timeout,
+                )
             )
             start_response = multiop_response.responses[0].start_workflow
             update_response = multiop_response.responses[1].update_workflow
diff --git a/temporalio/client/_interceptor.py b/temporalio/client/_interceptor.py
index 0e780146d..1f6dfa8b9 100644
--- a/temporalio/client/_interceptor.py
+++ b/temporalio/client/_interceptor.py
@@ -374,10 +374,20 @@ class UpdateWithStartStartWorkflowInput:
 
 @dataclass
 class StartWorkflowUpdateWithStartInput:
-    """Input for :py:meth:`OutboundInterceptor.start_update_with_start_workflow`."""
+    """Input for :py:meth:`OutboundInterceptor.start_update_with_start_workflow`.
+
+    The top-level ``rpc_metadata`` and ``rpc_timeout`` fields are authoritative
+    for the ``execute_multi_operation`` gRPC call.  The sub-inputs
+    (``start_workflow_input`` and ``update_workflow_input``) also carry their own
+    ``rpc_metadata`` / ``rpc_timeout`` for interceptor introspection, but those
+    values are **not** forwarded to the gRPC call.  Interceptors that wish to set
+    RPC metadata should modify :py:attr:`rpc_metadata` on this object.
+    """
 
     start_workflow_input: UpdateWithStartStartWorkflowInput
     update_workflow_input: UpdateWithStartUpdateWorkflowInput
+    rpc_metadata: Mapping[str, str | bytes]
+    rpc_timeout: timedelta | None
     _on_start: Callable[
         [temporalio.api.workflowservice.v1.StartWorkflowExecutionResponse], None
     ]
diff --git a/tests/worker/test_update_with_start.py b/tests/worker/test_update_with_start.py
index 4ed625960..2ceb5e91b 100644
--- a/tests/worker/test_update_with_start.py
+++ b/tests/worker/test_update_with_start.py
@@ -1104,3 +1104,84 @@ async def _do_update() -> Any:
         elif id_reuse_policy == WorkflowIDReusePolicy.REJECT_DUPLICATE:
             with pytest.raises(WorkflowAlreadyStartedError):
                 await _do_update()
+
+
+class MetadataCapturingInterceptor(Interceptor):
+    """Interceptor that sets rpc_metadata on update-with-start calls."""
+
+    def intercept_client(self, next: OutboundInterceptor) -> OutboundInterceptor:
+        return MetadataCapturingOutboundInterceptor(super().intercept_client(next))
+
+
+class MetadataCapturingOutboundInterceptor(OutboundInterceptor):
+    def __init__(self, next: OutboundInterceptor) -> None:
+        super().__init__(next)
+
+    async def start_update_with_start_workflow(
+        self, input: StartWorkflowUpdateWithStartInput
+    ) -> WorkflowUpdateHandle[Any]:
+        input.rpc_metadata = {
+            **input.rpc_metadata,
+            "test-header-key": "test-header-value",
+        }
+        return await super().start_update_with_start_workflow(input)
+
+
+# Verify fix for https://github.com/temporalio/sdk-python/issues/1582
+async def test_update_with_start_rpc_metadata_and_timeout_forwarded(client: Client):
+    """Test that rpc_metadata and rpc_timeout on StartWorkflowUpdateWithStartInput
+    are forwarded to the execute_multi_operation gRPC call."""
+    captured_metadata: dict[str, str | bytes] = {}
+    captured_timeout: list[timedelta | None] = []
+
+    class execute_multi_operation:
+        err = RPCError("intentional", RPCStatusCode.INTERNAL, b"")
+        err._grpc_status = temporalio.api.common.v1.GrpcStatus(details=[])
+
+        def __init__(self) -> None:  # type: ignore[reportMissingSuperCall]
+            pass
+
+        async def __call__(
+            self,
+            req: temporalio.api.workflowservice.v1.ExecuteMultiOperationRequest,
+            *,
+            retry: bool = False,
+            metadata: Mapping[str, str | bytes] = {},
+            timeout: timedelta | None = None,
+        ) -> temporalio.api.workflowservice.v1.ExecuteMultiOperationResponse:
+            captured_metadata.update(metadata)
+            captured_timeout.append(timeout)
+            raise self.err
+
+    interceptor = MetadataCapturingInterceptor()
+    intercepted_client = Client(
+        **{**client.config(), "interceptors": [interceptor]}  # type: ignore
+    )
+
+    with patch.object(
+        intercepted_client.workflow_service,
+        "execute_multi_operation",
+        execute_multi_operation(),
+    ):
+        start_workflow_operation = WithStartWorkflowOperation(
+            UpdateWithStartInterceptorWorkflow.run,
+            "wf-arg",
+            id=f"wf-{uuid.uuid4()}",
+            task_queue="tq",
+            id_conflict_policy=WorkflowIDConflictPolicy.FAIL,
+        )
+        with pytest.raises(RPCError):
+            await intercepted_client.start_update_with_start_workflow(
+                UpdateWithStartInterceptorWorkflow.my_update,
+                "update-arg",
+                start_workflow_operation=start_workflow_operation,
+                wait_for_stage=WorkflowUpdateStage.ACCEPTED,
+                rpc_metadata={"original-key": "original-value"},
+                rpc_timeout=timedelta(seconds=42),
+            )
+
+    # The interceptor should have added its metadata on top of the caller's
+    assert captured_metadata.get("test-header-key") == "test-header-value"
+    assert captured_metadata.get("original-key") == "original-value"
+    # The caller's timeout should have been forwarded
+    assert captured_timeout == [timedelta(seconds=42)]