diff --git a/federationsender/internal/api.go b/federationsender/internal/api.go
index 2a70f7ed3..49c537553 100644
--- a/federationsender/internal/api.go
+++ b/federationsender/internal/api.go
@@ -2,6 +2,7 @@ package internal
 
 import (
 	"context"
+	"sync"
 	"time"
 
 	"github.com/matrix-org/dendrite/federationsender/api"
@@ -23,6 +24,7 @@ type FederationSenderInternalAPI struct {
 	federation *gomatrixserverlib.FederationClient
 	keyRing    *gomatrixserverlib.KeyRing
 	queues     *queue.OutgoingQueues
+	joins      sync.Map // joins currently in progress
 }
 
 func NewFederationSenderInternalAPI(
diff --git a/federationsender/internal/perform.go b/federationsender/internal/perform.go
index f9d07e887..6aea296bd 100644
--- a/federationsender/internal/perform.go
+++ b/federationsender/internal/perform.go
@@ -37,12 +37,32 @@ func (r *FederationSenderInternalAPI) PerformDirectoryLookup(
 	return nil
 }
 
+type federatedJoin struct {
+	UserID string
+	RoomID string
+}
+
 // PerformJoinRequest implements api.FederationSenderInternalAPI
 func (r *FederationSenderInternalAPI) PerformJoin(
 	ctx context.Context,
 	request *api.PerformJoinRequest,
 	response *api.PerformJoinResponse,
 ) {
+	// Check that a join isn't already in progress for this user/room.
+	j := federatedJoin{request.UserID, request.RoomID}
+	if _, found := r.joins.Load(j); found {
+		response.LastError = &gomatrix.HTTPError{
+			Code: 429,
+			Message: `{
+				"errcode": "M_LIMIT_EXCEEDED",
+				"error": "There is already a federated join to this room in progress. Please wait for it to finish."
+			}`, // TODO: Why do none of our error types play nicely with each other?
+		}
+		return
+	}
+	r.joins.Store(j, nil)
+	defer r.joins.Delete(j)
+
 	// Look up the supported room versions.
 	var supportedVersions []gomatrixserverlib.RoomVersion
 	for version := range version.SupportedRoomVersions() {