diff --git a/changelog.d/12694.misc b/changelog.d/12694.misc
new file mode 100644
index 000000000..e1e956a51
--- /dev/null
+++ b/changelog.d/12694.misc
@@ -0,0 +1 @@
+Capture the `Deferred` for request cancellation in `_AsyncResource`.
diff --git a/synapse/http/server.py b/synapse/http/server.py
index 8c96f2196..4b4debc5c 100644
--- a/synapse/http/server.py
+++ b/synapse/http/server.py
@@ -344,7 +344,9 @@ class _AsyncResource(resource.Resource, metaclass=abc.ABCMeta):
 
     def render(self, request: SynapseRequest) -> int:
         """This gets called by twisted every time someone sends us a request."""
-        defer.ensureDeferred(self._async_render_wrapper(request))
+        request.render_deferred = defer.ensureDeferred(
+            self._async_render_wrapper(request)
+        )
         return NOT_DONE_YET
 
     @wrap_async_request_handler
diff --git a/synapse/http/site.py b/synapse/http/site.py
index f7f1c5704..eeec74b78 100644
--- a/synapse/http/site.py
+++ b/synapse/http/site.py
@@ -92,11 +92,12 @@ class SynapseRequest(Request):
         # we can't yet create the logcontext, as we don't know the method.
         self.logcontext: Optional[LoggingContext] = None
 
-        # The `Deferred` to cancel if the client disconnects early. Expected to be set
-        # by `Resource.render`.
+        # The `Deferred` to cancel if the client disconnects early and
+        # `is_render_cancellable` is set. Expected to be set by `Resource.render`.
         self.render_deferred: Optional["Deferred[None]"] = None
-        # A boolean indicating whether `_render_deferred` should be cancelled if the
-        # client disconnects early. Expected to be set during `Resource.render`.
+        # A boolean indicating whether `render_deferred` should be cancelled if the
+        # client disconnects early. Expected to be set by the coroutine started by
+        # `Resource.render`, if rendering is asynchronous.
         self.is_render_cancellable = False
 
         global _next_request_seq