diff --git a/syncapi/sync/requestpool.go b/syncapi/sync/requestpool.go
index 703340997..73c7ce9c3 100644
--- a/syncapi/sync/requestpool.go
+++ b/syncapi/sync/requestpool.go
@@ -337,12 +337,71 @@ func (rp *RequestPool) OnIncomingSyncRequest(req *http.Request, device *userapi.
 		}
 	}
 
+	if syncReq.Filter.Room.State.LazyLoadMembers {
+		for roomID, jr := range syncReq.Response.Rooms.Join {
+			jr.State.Events = rp.applyLazyLoadMembers(req.Context(), syncReq.Since.IsEmpty(), roomID, syncReq.Device.UserID, jr.Timeline.Events, jr.State.Events)
+			syncReq.Response.Rooms.Join[roomID] = jr
+		}
+	}
+
 	return util.JSONResponse{
 		Code: http.StatusOK,
 		JSON: syncReq.Response,
 	}
 }
 
+func (rp *RequestPool) applyLazyLoadMembers(
+	ctx context.Context, isInitial bool, roomID, userID string, timelineEvents, stateEvents []gomatrixserverlib.ClientEvent,
+) []gomatrixserverlib.ClientEvent {
+	if len(stateEvents) == 0 || len(timelineEvents) == 0 {
+		return stateEvents
+	}
+	logrus.Debugf("before stateEvents: %d", len(stateEvents))
+
+	// First, get a list of users we need in the response
+	requiredUsers := make(map[string]bool)
+	if isInitial {
+		requiredUsers[userID] = true
+	}
+	for _, ev := range timelineEvents {
+		requiredUsers[ev.Sender] = true
+	}
+	// Filter out users who didn't send an event
+	newState := []gomatrixserverlib.ClientEvent{}
+	membershipEvents := []gomatrixserverlib.ClientEvent{}
+	for _, event := range stateEvents {
+		if event.Type != gomatrixserverlib.MRoomMember {
+			newState = append(newState, event)
+		} else {
+			// did the user send an event?
+			if requiredUsers[event.Sender] {
+				membershipEvents = append(membershipEvents, event)
+				delete(requiredUsers, event.Sender)
+			}
+		}
+	}
+	// Get all remaining users in the list
+	membershipToUser := make(map[string]*gomatrixserverlib.HeaderedEvent)
+	for userID := range requiredUsers {
+		membership, err := rp.db.GetStateEvent(ctx, roomID, gomatrixserverlib.MRoomMember, userID)
+		if err != nil {
+			util.GetLogger(ctx).WithError(err).Error("failed to get membership event for user")
+			continue
+		}
+		if membership != nil {
+			membershipToUser[userID] = membership
+		}
+	}
+	// Convert HeaderedEvent to ClientEvent
+	for _, evt := range membershipToUser {
+		newState = append(newState, gomatrixserverlib.HeaderedToClientEvent(evt, gomatrixserverlib.FormatSync))
+	}
+
+	result := append(newState, membershipEvents...)
+	logrus.Debugf("after stateEvents: %d", len(result))
+	return result
+}
+
 func (rp *RequestPool) OnIncomingKeyChangeRequest(req *http.Request, device *userapi.Device) util.JSONResponse {
 	from := req.URL.Query().Get("from")
 	to := req.URL.Query().Get("to")