diff --git a/clientapi/routing/redaction.go b/clientapi/routing/redaction.go
index 01ea818a..e8d14ce3 100644
--- a/clientapi/routing/redaction.go
+++ b/clientapi/routing/redaction.go
@@ -22,6 +22,7 @@ import (
 	"github.com/matrix-org/dendrite/clientapi/httputil"
 	"github.com/matrix-org/dendrite/clientapi/jsonerror"
 	"github.com/matrix-org/dendrite/internal/eventutil"
+	"github.com/matrix-org/dendrite/internal/transactions"
 	roomserverAPI "github.com/matrix-org/dendrite/roomserver/api"
 	"github.com/matrix-org/dendrite/setup/config"
 	userapi "github.com/matrix-org/dendrite/userapi/api"
@@ -40,12 +41,21 @@ type redactionResponse struct {
 func SendRedaction(
 	req *http.Request, device *userapi.Device, roomID, eventID string, cfg *config.ClientAPI,
 	rsAPI roomserverAPI.RoomserverInternalAPI,
+	txnID *string,
+	txnCache *transactions.Cache,
 ) util.JSONResponse {
 	resErr := checkMemberInRoom(req.Context(), rsAPI, device.UserID, roomID)
 	if resErr != nil {
 		return *resErr
 	}
 
+	if txnID != nil {
+		// Try to fetch response from transactionsCache
+		if res, ok := txnCache.FetchTransaction(device.AccessToken, *txnID); ok {
+			return *res
+		}
+	}
+
 	ev := roomserverAPI.GetEvent(req.Context(), rsAPI, eventID)
 	if ev == nil {
 		return util.JSONResponse{
@@ -124,10 +134,18 @@ func SendRedaction(
 		util.GetLogger(req.Context()).WithError(err).Errorf("failed to SendEvents")
 		return jsonerror.InternalServerError()
 	}
-	return util.JSONResponse{
+
+	res := util.JSONResponse{
 		Code: 200,
 		JSON: redactionResponse{
 			EventID: e.EventID(),
 		},
 	}
+
+	// Add response to transactionsCache
+	if txnID != nil {
+		txnCache.AddTransaction(device.AccessToken, *txnID, &res)
+	}
+
+	return res
 }
diff --git a/clientapi/routing/routing.go b/clientapi/routing/routing.go
index 37d825b8..f370b4f8 100644
--- a/clientapi/routing/routing.go
+++ b/clientapi/routing/routing.go
@@ -479,7 +479,7 @@ func Setup(
 			if err != nil {
 				return util.ErrorResponse(err)
 			}
-			return SendRedaction(req, device, vars["roomID"], vars["eventID"], cfg, rsAPI)
+			return SendRedaction(req, device, vars["roomID"], vars["eventID"], cfg, rsAPI, nil, nil)
 		}),
 	).Methods(http.MethodPost, http.MethodOptions)
 	v3mux.Handle("/rooms/{roomID}/redact/{eventID}/{txnId}",
@@ -488,7 +488,8 @@ func Setup(
 			if err != nil {
 				return util.ErrorResponse(err)
 			}
-			return SendRedaction(req, device, vars["roomID"], vars["eventID"], cfg, rsAPI)
+			txnID := vars["txnId"]
+			return SendRedaction(req, device, vars["roomID"], vars["eventID"], cfg, rsAPI, &txnID, transactionsCache)
 		}),
 	).Methods(http.MethodPut, http.MethodOptions)
 
diff --git a/sytest-whitelist b/sytest-whitelist
index 5d67aee6..91304bd7 100644
--- a/sytest-whitelist
+++ b/sytest-whitelist
@@ -713,4 +713,5 @@ Presence can be set from sync
 /state returns M_NOT_FOUND for a rejected message event
 /state_ids returns M_NOT_FOUND for a rejected message event
 /state returns M_NOT_FOUND for a rejected state event
-/state_ids returns M_NOT_FOUND for a rejected state event
\ No newline at end of file
+/state_ids returns M_NOT_FOUND for a rejected state event
+PUT /rooms/:room_id/redact/:event_id/:txn_id is idempotent
\ No newline at end of file