diff --git a/syncapi/storage/syncserver.go b/syncapi/storage/syncserver.go
index 418cf5b78..3f3101670 100644
--- a/syncapi/storage/syncserver.go
+++ b/syncapi/storage/syncserver.go
@@ -250,7 +250,11 @@ func (d *SyncServerDatasource) addPDUDeltaToResponse(
 	// This works out what the 'state' key should be for each room as well as which membership block
 	// to put the room into.
 	var deltas []stateDelta
-	deltas, err = d.getStateDeltas(ctx, &device, txn, fromPos, toPos, device.UserID, wantFullState)
+	if !wantFullState {
+		deltas, err = d.getStateDeltas(ctx, &device, txn, fromPos, toPos, device.UserID)
+	} else {
+		deltas, err = d.getStateDeltasForFullStateSync(ctx, &device, txn, fromPos, toPos, device.UserID)
+	}
 	if err != nil {
 		return nil, err
 	}
@@ -725,7 +729,7 @@ func (d *SyncServerDatasource) fetchMissingStateEvents(
 
 func (d *SyncServerDatasource) getStateDeltas(
 	ctx context.Context, device *authtypes.Device, txn *sql.Tx,
-	fromPos, toPos int64, userID string, wantFullState bool,
+	fromPos, toPos int64, userID string,
 ) ([]stateDelta, error) {
 	// Implement membership change algorithm: https://github.com/matrix-org/synapse/blob/v0.19.3/synapse/handlers/sync.py#L821
 	// - Get membership list changes for this user in this sync response
@@ -756,13 +760,6 @@ func (d *SyncServerDatasource) getStateDeltas(
 			//       the timeline.
 			if membership := getMembershipFromEvent(&ev.Event, userID); membership != "" {
 				if membership == "join" {
-					if wantFullState {
-						// If full_state=true, we need to return full state for
-						// ALL joined rooms. So we skip handling individual
-						// rooms here and do this later.
-						continue
-					}
-
 					// send full room state down instead of a delta
 					var s []streamEvent
 					s, err = d.currentStateStreamEventsForRoom(ctx, txn, roomID)
@@ -790,21 +787,9 @@ func (d *SyncServerDatasource) getStateDeltas(
 		return nil, err
 	}
 	for _, joinedRoomID := range joinedRoomIDs {
-		var toAdd []streamEvent
-
-		if wantFullState {
-			s, err := d.currentStateStreamEventsForRoom(ctx, txn, joinedRoomID)
-			if err != nil {
-				return nil, err
-			}
-			toAdd = s
-		} else {
-			toAdd = state[joinedRoomID]
-		}
-
 		deltas = append(deltas, stateDelta{
 			membership:  "join",
-			stateEvents: streamEventsToEvents(device, toAdd),
+			stateEvents: streamEventsToEvents(device, state[joinedRoomID]),
 			roomID:      joinedRoomID,
 		})
 	}
@@ -812,6 +797,65 @@ func (d *SyncServerDatasource) getStateDeltas(
 	return deltas, nil
 }
 
+// getStateDeltasForFullStateSync is a variant of getStateDeltas used for /sync
+// requests with full_state=true.
+// Fetches full state for all joined rooms and uses selectStateInRange to get
+// updates for other rooms.
+func (d *SyncServerDatasource) getStateDeltasForFullStateSync(
+	ctx context.Context, device *authtypes.Device, txn *sql.Tx,
+	fromPos, toPos int64, userID string,
+) ([]stateDelta, error) {
+	joinedRoomIDs, err := d.roomstate.selectRoomIDsWithMembership(ctx, txn, userID, "join")
+	if err != nil {
+		return nil, err
+	}
+
+	// Use a reasonable initial capacity
+	deltas := make([]stateDelta, 0, len(joinedRoomIDs))
+
+	// Add full states for all joined rooms
+	for _, joinedRoomID := range joinedRoomIDs {
+		s, stateErr := d.currentStateStreamEventsForRoom(ctx, txn, joinedRoomID)
+		if stateErr != nil {
+			return nil, stateErr
+		}
+		deltas = append(deltas, stateDelta{
+			membership:  "join",
+			stateEvents: streamEventsToEvents(device, s),
+			roomID:      joinedRoomID,
+		})
+	}
+
+	// get all the state events ever between these two positions
+	stateNeeded, eventMap, err := d.events.selectStateInRange(ctx, txn, fromPos, toPos)
+	if err != nil {
+		return nil, err
+	}
+	state, err := d.fetchStateEvents(ctx, txn, stateNeeded, eventMap)
+	if err != nil {
+		return nil, err
+	}
+
+	for roomID, stateStreamEvents := range state {
+		for _, ev := range stateStreamEvents {
+			if membership := getMembershipFromEvent(&ev.Event, userID); membership != "" {
+				if membership != "join" { // We've already added full state for all joined rooms above.
+					deltas = append(deltas, stateDelta{
+						membership:    membership,
+						membershipPos: ev.streamPosition,
+						stateEvents:   streamEventsToEvents(device, stateStreamEvents),
+						roomID:        roomID,
+					})
+				}
+
+				break
+			}
+		}
+	}
+
+	return deltas, nil
+}
+
 func (d *SyncServerDatasource) currentStateStreamEventsForRoom(
 	ctx context.Context, txn *sql.Tx, roomID string,
 ) ([]streamEvent, error) {