diff --git a/synapse/rest/client/v1/room.py b/synapse/rest/client/v1/room.py
index 2ebf5e59a0f4..728e3df0e3e4 100644
--- a/synapse/rest/client/v1/room.py
+++ b/synapse/rest/client/v1/room.py
@@ -609,6 +609,10 @@ def on_POST(self, request, room_id, membership_action, txn_id=None):
                 raise SynapseError(400, "Missing user_id key.")
             target = UserID.from_string(content["user_id"])
 
+        event_content = None
+        if 'reason' in content and membership_action in ['kick', 'ban']:
+            event_content = {'reason': content['reason']}
+
         yield self.handlers.room_member_handler.update_membership(
             requester=requester,
             target=target,
@@ -616,6 +620,7 @@ def on_POST(self, request, room_id, membership_action, txn_id=None):
             action=membership_action,
             txn_id=txn_id,
             third_party_signed=content.get("third_party_signed", None),
+            content=event_content,
         )
 
         defer.returnValue((200, {}))