diff --git a/src/github.com/matrix-org/dendrite/clientapi/clientapi.go b/src/github.com/matrix-org/dendrite/clientapi/clientapi.go
index 11177ab08..6f31ca755 100644
--- a/src/github.com/matrix-org/dendrite/clientapi/clientapi.go
+++ b/src/github.com/matrix-org/dendrite/clientapi/clientapi.go
@@ -21,6 +21,7 @@ import (
 	"github.com/matrix-org/dendrite/clientapi/producers"
 	"github.com/matrix-org/dendrite/clientapi/routing"
 	"github.com/matrix-org/dendrite/common/basecomponent"
+	"github.com/matrix-org/dendrite/common/transactions"
 	"github.com/matrix-org/dendrite/roomserver/api"
 	"github.com/matrix-org/gomatrixserverlib"
 	"github.com/sirupsen/logrus"
@@ -37,6 +38,7 @@ func SetupClientAPIComponent(
 	aliasAPI api.RoomserverAliasAPI,
 	inputAPI api.RoomserverInputAPI,
 	queryAPI api.RoomserverQueryAPI,
+	transactionsCache *transactions.Cache,
 ) {
 	roomserverProducer := producers.NewRoomserverProducer(inputAPI)
 
@@ -62,5 +64,6 @@ func SetupClientAPIComponent(
 		queryAPI, aliasAPI, accountsDB, deviceDB,
 		federation, *keyRing,
 		userUpdateProducer, syncProducer,
+		transactionsCache,
 	)
 }
diff --git a/src/github.com/matrix-org/dendrite/clientapi/routing/routing.go b/src/github.com/matrix-org/dendrite/clientapi/routing/routing.go
index a746fa095..8e6b2b0a2 100644
--- a/src/github.com/matrix-org/dendrite/clientapi/routing/routing.go
+++ b/src/github.com/matrix-org/dendrite/clientapi/routing/routing.go
@@ -27,6 +27,7 @@ import (
 	"github.com/matrix-org/dendrite/clientapi/producers"
 	"github.com/matrix-org/dendrite/common"
 	"github.com/matrix-org/dendrite/common/config"
+	"github.com/matrix-org/dendrite/common/transactions"
 	"github.com/matrix-org/dendrite/roomserver/api"
 	"github.com/matrix-org/gomatrixserverlib"
 	"github.com/matrix-org/util"
@@ -48,6 +49,7 @@ func Setup(
 	keyRing gomatrixserverlib.KeyRing,
 	userUpdateProducer *producers.UserUpdateProducer,
 	syncProducer *producers.SyncAPIProducer,
+	transactionsCache *transactions.Cache,
 ) {
 
 	apiMux.Handle("/_matrix/client/versions",
@@ -92,14 +94,15 @@ func Setup(
 	r0mux.Handle("/rooms/{roomID}/send/{eventType}",
 		common.MakeAuthAPI("send_message", deviceDB, func(req *http.Request, device *authtypes.Device) util.JSONResponse {
 			vars := mux.Vars(req)
-			return SendEvent(req, device, vars["roomID"], vars["eventType"], nil, nil, cfg, queryAPI, producer)
+			return SendEvent(req, device, vars["roomID"], vars["eventType"], nil, nil, cfg, queryAPI, producer, nil)
 		}),
 	).Methods(http.MethodPost, http.MethodOptions)
 	r0mux.Handle("/rooms/{roomID}/send/{eventType}/{txnID}",
 		common.MakeAuthAPI("send_message", deviceDB, func(req *http.Request, device *authtypes.Device) util.JSONResponse {
 			vars := mux.Vars(req)
 			txnID := vars["txnID"]
-			return SendEvent(req, device, vars["roomID"], vars["eventType"], &txnID, nil, cfg, queryAPI, producer)
+			return SendEvent(req, device, vars["roomID"], vars["eventType"], &txnID,
+				nil, cfg, queryAPI, producer, transactionsCache)
 		}),
 	).Methods(http.MethodPut, http.MethodOptions)
 	r0mux.Handle("/rooms/{roomID}/state/{eventType:[^/]+/?}",
@@ -111,14 +114,14 @@ func Setup(
 			if strings.HasSuffix(eventType, "/") {
 				eventType = eventType[:len(eventType)-1]
 			}
-			return SendEvent(req, device, vars["roomID"], eventType, nil, &emptyString, cfg, queryAPI, producer)
+			return SendEvent(req, device, vars["roomID"], eventType, nil, &emptyString, cfg, queryAPI, producer, nil)
 		}),
 	).Methods(http.MethodPut, http.MethodOptions)
 	r0mux.Handle("/rooms/{roomID}/state/{eventType}/{stateKey}",
 		common.MakeAuthAPI("send_message", deviceDB, func(req *http.Request, device *authtypes.Device) util.JSONResponse {
 			vars := mux.Vars(req)
 			stateKey := vars["stateKey"]
-			return SendEvent(req, device, vars["roomID"], vars["eventType"], nil, &stateKey, cfg, queryAPI, producer)
+			return SendEvent(req, device, vars["roomID"], vars["eventType"], nil, &stateKey, cfg, queryAPI, producer, nil)
 		}),
 	).Methods(http.MethodPut, http.MethodOptions)
 
diff --git a/src/github.com/matrix-org/dendrite/clientapi/routing/sendevent.go b/src/github.com/matrix-org/dendrite/clientapi/routing/sendevent.go
index f47aa0c2d..46ede5983 100644
--- a/src/github.com/matrix-org/dendrite/clientapi/routing/sendevent.go
+++ b/src/github.com/matrix-org/dendrite/clientapi/routing/sendevent.go
@@ -23,6 +23,7 @@ import (
 	"github.com/matrix-org/dendrite/clientapi/producers"
 	"github.com/matrix-org/dendrite/common"
 	"github.com/matrix-org/dendrite/common/config"
+	"github.com/matrix-org/dendrite/common/transactions"
 	"github.com/matrix-org/dendrite/roomserver/api"
 	"github.com/matrix-org/gomatrixserverlib"
 	"github.com/matrix-org/util"
@@ -45,7 +46,15 @@ func SendEvent(
 	cfg config.Dendrite,
 	queryAPI api.RoomserverQueryAPI,
 	producer *producers.RoomserverProducer,
+	txnCache *transactions.Cache,
 ) util.JSONResponse {
+	if txnID != nil {
+		// Try to fetch response from transactionsCache
+		if res, ok := txnCache.FetchTransaction(*txnID); ok {
+			return *res
+		}
+	}
+
 	// parse the incoming http request
 	userID := device.UserID
 	var r map[string]interface{} // must be a JSON object
@@ -105,8 +114,14 @@ func SendEvent(
 		return httputil.LogThenError(req, err)
 	}
 
-	return util.JSONResponse{
+	res := util.JSONResponse{
 		Code: http.StatusOK,
 		JSON: sendEventResponse{e.EventID()},
 	}
+	// Add response to transactionsCache
+	if txnID != nil {
+		txnCache.AddTransaction(*txnID, &res)
+	}
+
+	return res
 }
diff --git a/src/github.com/matrix-org/dendrite/cmd/dendrite-client-api-server/main.go b/src/github.com/matrix-org/dendrite/cmd/dendrite-client-api-server/main.go
index 2845eb364..be04a89e2 100644
--- a/src/github.com/matrix-org/dendrite/cmd/dendrite-client-api-server/main.go
+++ b/src/github.com/matrix-org/dendrite/cmd/dendrite-client-api-server/main.go
@@ -18,6 +18,7 @@ import (
 	"github.com/matrix-org/dendrite/clientapi"
 	"github.com/matrix-org/dendrite/common/basecomponent"
 	"github.com/matrix-org/dendrite/common/keydb"
+	"github.com/matrix-org/dendrite/common/transactions"
 )
 
 func main() {
@@ -33,10 +34,11 @@ func main() {
 	keyRing := keydb.CreateKeyRing(federation.Client, keyDB)
 
 	alias, input, query := base.CreateHTTPRoomserverAPIs()
+	cache := transactions.New()
 
 	clientapi.SetupClientAPIComponent(
 		base, deviceDB, accountDB, federation, &keyRing,
-		alias, input, query,
+		alias, input, query, cache,
 	)
 
 	base.SetupAndServeHTTP(string(base.Cfg.Listen.ClientAPI))
diff --git a/src/github.com/matrix-org/dendrite/cmd/dendrite-monolith-server/main.go b/src/github.com/matrix-org/dendrite/cmd/dendrite-monolith-server/main.go
index 89005c9d3..c00154a78 100644
--- a/src/github.com/matrix-org/dendrite/cmd/dendrite-monolith-server/main.go
+++ b/src/github.com/matrix-org/dendrite/cmd/dendrite-monolith-server/main.go
@@ -19,6 +19,7 @@ import (
 	"net/http"
 
 	"github.com/matrix-org/dendrite/common/keydb"
+	"github.com/matrix-org/dendrite/common/transactions"
 
 	"github.com/matrix-org/dendrite/clientapi"
 	"github.com/matrix-org/dendrite/common"
@@ -52,7 +53,11 @@ func main() {
 
 	alias, input, query := roomserver.SetupRoomServerComponent(base)
 
-	clientapi.SetupClientAPIComponent(base, deviceDB, accountDB, federation, &keyRing, alias, input, query)
+	clientapi.SetupClientAPIComponent(
+		base, deviceDB, accountDB,
+		federation, &keyRing, alias, input, query,
+		transactions.New(),
+	)
 	federationapi.SetupFederationAPIComponent(base, accountDB, federation, &keyRing, alias, input, query)
 	federationsender.SetupFederationSenderComponent(base, federation, query)
 	mediaapi.SetupMediaAPIComponent(base, deviceDB)