diff --git a/federationapi/routing/routing.go b/federationapi/routing/routing.go
index b579ae1fa..ce018904f 100644
--- a/federationapi/routing/routing.go
+++ b/federationapi/routing/routing.go
@@ -21,6 +21,7 @@ import (
 	"github.com/matrix-org/dendrite/clientapi/jsonerror"
 	eduserverAPI "github.com/matrix-org/dendrite/eduserver/api"
 	federationSenderAPI "github.com/matrix-org/dendrite/federationsender/api"
+	"github.com/matrix-org/dendrite/internal"
 	"github.com/matrix-org/dendrite/internal/httputil"
 	keyserverAPI "github.com/matrix-org/dendrite/keyserver/api"
 	roomserverAPI "github.com/matrix-org/dendrite/roomserver/api"
@@ -92,12 +93,13 @@ func Setup(
 	v2keysmux.Handle("/query", notaryKeys).Methods(http.MethodPost)
 	v2keysmux.Handle("/query/{serverName}/{keyID}", notaryKeys).Methods(http.MethodGet)
 
+	mu := internal.NewMutexByRoom()
 	v1fedmux.Handle("/send/{txnID}", httputil.MakeFedAPI(
 		"federation_send", cfg.Matrix.ServerName, keys, wakeup,
 		func(httpReq *http.Request, request *gomatrixserverlib.FederationRequest, vars map[string]string) util.JSONResponse {
 			return Send(
 				httpReq, request, gomatrixserverlib.TransactionID(vars["txnID"]),
-				cfg, rsAPI, eduAPI, keyAPI, keys, federation,
+				cfg, rsAPI, eduAPI, keyAPI, keys, federation, mu,
 			)
 		},
 	)).Methods(http.MethodPut, http.MethodOptions)
diff --git a/federationapi/routing/send.go b/federationapi/routing/send.go
index 231a16863..b48d6c0b8 100644
--- a/federationapi/routing/send.go
+++ b/federationapi/routing/send.go
@@ -26,6 +26,7 @@ import (
 	"github.com/getsentry/sentry-go"
 	"github.com/matrix-org/dendrite/clientapi/jsonerror"
 	eduserverAPI "github.com/matrix-org/dendrite/eduserver/api"
+	"github.com/matrix-org/dendrite/internal"
 	keyapi "github.com/matrix-org/dendrite/keyserver/api"
 	"github.com/matrix-org/dendrite/roomserver/api"
 	"github.com/matrix-org/dendrite/setup/config"
@@ -98,6 +99,7 @@ func Send(
 	keyAPI keyapi.KeyInternalAPI,
 	keys gomatrixserverlib.JSONVerifier,
 	federation *gomatrixserverlib.FederationClient,
+	mu *internal.MutexByRoom,
 ) util.JSONResponse {
 	t := txnReq{
 		rsAPI:      rsAPI,
@@ -107,6 +109,7 @@ func Send(
 		haveEvents: make(map[string]*gomatrixserverlib.HeaderedEvent),
 		newEvents:  make(map[string]bool),
 		keyAPI:     keyAPI,
+		roomsMu:    mu,
 	}
 
 	var txnEvents struct {
@@ -163,6 +166,7 @@ type txnReq struct {
 	federation   txnFederationClient
 	servers      []gomatrixserverlib.ServerName
 	serversMutex sync.RWMutex
+	roomsMu      *internal.MutexByRoom
 	// local cache of events for auth checks, etc - this may include events
 	// which the roomserver is unaware of.
 	haveEvents map[string]*gomatrixserverlib.HeaderedEvent
@@ -494,6 +498,8 @@ func (t *txnReq) getServers(ctx context.Context, roomID string) []gomatrixserver
 }
 
 func (t *txnReq) processEvent(ctx context.Context, e *gomatrixserverlib.Event) error {
+	t.roomsMu.Lock(e.RoomID())
+	defer t.roomsMu.Unlock(e.RoomID())
 	logger := util.GetLogger(ctx).WithField("event_id", e.EventID()).WithField("room_id", e.RoomID())
 	t.work = "" // reset from previous event
 
diff --git a/federationapi/routing/send_test.go b/federationapi/routing/send_test.go
index 8bdf54c4a..b14cbd35a 100644
--- a/federationapi/routing/send_test.go
+++ b/federationapi/routing/send_test.go
@@ -9,6 +9,7 @@ import (
 	"time"
 
 	eduAPI "github.com/matrix-org/dendrite/eduserver/api"
+	"github.com/matrix-org/dendrite/internal"
 	"github.com/matrix-org/dendrite/internal/test"
 	"github.com/matrix-org/dendrite/roomserver/api"
 	"github.com/matrix-org/gomatrixserverlib"
@@ -370,6 +371,7 @@ func mustCreateTransaction(rsAPI api.RoomserverInternalAPI, fedClient txnFederat
 		federation: fedClient,
 		haveEvents: make(map[string]*gomatrixserverlib.HeaderedEvent),
 		newEvents:  make(map[string]bool),
+		roomsMu:    internal.NewMutexByRoom(),
 	}
 	t.PDUs = pdus
 	t.Origin = testOrigin
diff --git a/internal/mutex.go b/internal/mutex.go
new file mode 100644
index 000000000..3d36cdac9
--- /dev/null
+++ b/internal/mutex.go
@@ -0,0 +1,38 @@
+package internal
+
+import "sync"
+
+type MutexByRoom struct {
+	mu       *sync.Mutex // protects the map
+	roomToMu map[string]*sync.Mutex
+}
+
+func NewMutexByRoom() *MutexByRoom {
+	return &MutexByRoom{
+		mu:       &sync.Mutex{},
+		roomToMu: make(map[string]*sync.Mutex),
+	}
+}
+
+func (m *MutexByRoom) Lock(roomID string) {
+	m.mu.Lock()
+	roomMu := m.roomToMu[roomID]
+	if roomMu == nil {
+		roomMu = &sync.Mutex{}
+	}
+	m.roomToMu[roomID] = roomMu
+	m.mu.Unlock()
+	// don't lock inside m.mu else we can deadlock
+	roomMu.Lock()
+}
+
+func (m *MutexByRoom) Unlock(roomID string) {
+	m.mu.Lock()
+	roomMu := m.roomToMu[roomID]
+	if roomMu == nil {
+		panic("MutexByRoom: Unlock before Lock")
+	}
+	m.mu.Unlock()
+
+	roomMu.Unlock()
+}