diff --git a/synapse/handlers/read_marker.py b/synapse/handlers/read_marker.py
index 05122fd5a..0d9d6fe93 100644
--- a/synapse/handlers/read_marker.py
+++ b/synapse/handlers/read_marker.py
@@ -13,9 +13,10 @@
 # limitations under the License.
 
 import logging
-from typing import TYPE_CHECKING
+from typing import TYPE_CHECKING, Optional
 
 from synapse.util.async_helpers import Linearizer
+from synapse.types import JsonDict
 
 if TYPE_CHECKING:
     from synapse.server import HomeServer
@@ -31,7 +32,11 @@ class ReadMarkerHandler:
         self.read_marker_linearizer = Linearizer(name="read_marker")
 
     async def received_client_read_marker(
-        self, room_id: str, user_id: str, event_id: str
+        self,
+        room_id: str,
+        user_id: str,
+        event_id: str,
+        extra_content: Optional[JsonDict] = None,
     ) -> None:
         """Updates the read marker for a given user in a given room if the event ID given
         is ahead in the stream relative to the current read marker.
@@ -54,7 +59,7 @@ class ReadMarkerHandler:
                 )
 
             if should_update:
-                content = {"event_id": event_id}
+                content = {"event_id": event_id, **(extra_content or {})}
                 await self.account_data_handler.add_account_data_to_room(
                     user_id, room_id, "m.fully_read", content
                 )
diff --git a/synapse/handlers/receipts.py b/synapse/handlers/receipts.py
index 04c61ae3d..c9be10a57 100644
--- a/synapse/handlers/receipts.py
+++ b/synapse/handlers/receipts.py
@@ -161,6 +161,7 @@ class ReceiptsHandler:
         user_id: str,
         event_id: str,
         thread_id: Optional[str],
+        extra_content: Optional[JsonDict] = None,
     ) -> None:
         """Called when a client tells us a local user has read up to the given
         event_id in the room.
@@ -171,7 +172,7 @@ class ReceiptsHandler:
             user_id=user_id,
             event_ids=[event_id],
             thread_id=thread_id,
-            data={"ts": int(self.clock.time_msec())},
+            data={"ts": int(self.clock.time_msec()), **(extra_content or {})},
         )
 
         is_new = await self._handle_new_receipts([receipt])
diff --git a/synapse/rest/client/read_marker.py b/synapse/rest/client/read_marker.py
index 852838515..e438f8f73 100644
--- a/synapse/rest/client/read_marker.py
+++ b/synapse/rest/client/read_marker.py
@@ -70,12 +70,16 @@ class ReadMarkerRestServlet(RestServlet):
             # TODO Add validation to reject non-string event IDs.
             if not event_id:
                 continue
+            extra_content = body.get(
+                receipt_type.replace("m.", "com.beeper.") + ".extra", None
+            )
 
             if receipt_type == ReceiptTypes.FULLY_READ:
                 await self.read_marker_handler.received_client_read_marker(
                     room_id,
                     user_id=requester.user.to_string(),
                     event_id=event_id,
+                    extra_content=extra_content,
                 )
             else:
                 await self.receipts_handler.received_client_receipt(
@@ -85,6 +89,7 @@ class ReadMarkerRestServlet(RestServlet):
                     event_id=event_id,
                     # Setting the thread ID is not possible with the /read_markers endpoint.
                     thread_id=None,
+                    extra_content=extra_content,
                 )
 
         return 200, {}
diff --git a/synapse/rest/client/receipts.py b/synapse/rest/client/receipts.py
index 28b7d30ea..3ab42b53d 100644
--- a/synapse/rest/client/receipts.py
+++ b/synapse/rest/client/receipts.py
@@ -65,7 +65,7 @@ class ReceiptRestServlet(RestServlet):
                 f"Receipt type must be {', '.join(self._known_receipt_types)}",
             )
 
-        body = parse_json_object_from_request(request)
+        body = parse_json_object_from_request(request, allow_empty_body=False)
 
         # Pull the thread ID, if one exists.
         thread_id = None
@@ -100,6 +100,7 @@ class ReceiptRestServlet(RestServlet):
                 room_id,
                 user_id=requester.user.to_string(),
                 event_id=event_id,
+                extra_content=body,
             )
         else:
             await self.receipts_handler.received_client_receipt(
@@ -108,6 +109,7 @@ class ReceiptRestServlet(RestServlet):
                 user_id=requester.user.to_string(),
                 event_id=event_id,
                 thread_id=thread_id,
+                extra_content=body,
             )
 
         return 200, {}