diff --git a/federationsender/internal/api.go b/federationsender/internal/api.go
index 2a70f7ed..49c53755 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 a0abf7ff..6aea296b 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() {
@@ -186,27 +206,47 @@ func (r *FederationSenderInternalAPI) performJoinUsingServer(
 	}
 	r.statistics.ForServer(serverName).Success()
 
-	// Check that the send_join response was valid.
-	joinCtx := perform.JoinContext(r.federation, r.keyRing)
-	respState, err := joinCtx.CheckSendJoinResponse(
-		ctx, event, serverName, respMakeJoin, respSendJoin,
-	)
-	if err != nil {
-		return fmt.Errorf("joinCtx.CheckSendJoinResponse: %w", err)
-	}
+	// Process the join response in a goroutine. The idea here is
+	// that we'll try and wait for as long as possible for the work
+	// to complete, but if the client does give up waiting, we'll
+	// still continue to process the join anyway so that we don't
+	// waste the effort.
+	var cancel context.CancelFunc
+	ctx, cancel = context.WithCancel(context.Background())
+	go func() {
+		defer cancel()
 
-	// If we successfully performed a send_join above then the other
-	// server now thinks we're a part of the room. Send the newly
-	// returned state to the roomserver to update our local view.
-	if err = roomserverAPI.SendEventWithRewrite(
-		ctx, r.rsAPI,
-		respState,
-		event.Headered(respMakeJoin.RoomVersion),
-		nil,
-	); err != nil {
-		return fmt.Errorf("r.producer.SendEventWithState: %w", err)
-	}
+		// Check that the send_join response was valid.
+		joinCtx := perform.JoinContext(r.federation, r.keyRing)
+		respState, err := joinCtx.CheckSendJoinResponse(
+			ctx, event, serverName, respMakeJoin, respSendJoin,
+		)
+		if err != nil {
+			logrus.WithFields(logrus.Fields{
+				"room_id": roomID,
+				"user_id": userID,
+			}).WithError(err).Error("Failed to process room join response")
+			return
+		}
 
+		// If we successfully performed a send_join above then the other
+		// server now thinks we're a part of the room. Send the newly
+		// returned state to the roomserver to update our local view.
+		if err = roomserverAPI.SendEventWithRewrite(
+			ctx, r.rsAPI,
+			respState,
+			event.Headered(respMakeJoin.RoomVersion),
+			nil,
+		); err != nil {
+			logrus.WithFields(logrus.Fields{
+				"room_id": roomID,
+				"user_id": userID,
+			}).WithError(err).Error("Failed to send room join response to roomserver")
+			return
+		}
+	}()
+
+	<-ctx.Done()
 	return nil
 }