diff --git a/synapse/handlers/read_marker.py b/synapse/handlers/read_marker.py
index 58593e570..29562d9e3 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 +32,7 @@ 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 +55,7 @@ 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/rest/client/read_marker.py b/synapse/rest/client/read_marker.py
index 43c04fac6..bee56c0b1 100644
--- a/synapse/rest/client/read_marker.py
+++ b/synapse/rest/client/read_marker.py
@@ -49,6 +49,7 @@ class ReadMarkerRestServlet(RestServlet):
 
         body = parse_json_object_from_request(request)
         read_event_id = body.get("m.read", None)
+        read_extra = body.get("com.beeper.read.extra", None)
         hidden = body.get(ReadReceiptEventFields.MSC2285_HIDDEN, False)
 
         if not isinstance(hidden, bool):
@@ -66,14 +67,17 @@ class ReadMarkerRestServlet(RestServlet):
                 user_id=requester.user.to_string(),
                 event_id=read_event_id,
                 hidden=hidden,
+                extra_content=read_extra,
             )
 
         read_marker_event_id = body.get("m.fully_read", None)
+        read_marker_extra = body.get("com.beeper.fully_read.extra", None)
         if read_marker_event_id:
             await self.read_marker_handler.received_client_read_marker(
                 room_id,
                 user_id=requester.user.to_string(),
                 event_id=read_marker_event_id,
+                extra_content=read_marker_extra,
             )
 
         return 200, {}