diff --git a/clientapi/routing/joinroom.go b/clientapi/routing/joinroom.go
index c10113574..578aaec56 100644
--- a/clientapi/routing/joinroom.go
+++ b/clientapi/routing/joinroom.go
@@ -16,9 +16,11 @@ package routing
 
 import (
 	"net/http"
+	"time"
 
 	"github.com/matrix-org/dendrite/clientapi/auth/authtypes"
 	"github.com/matrix-org/dendrite/clientapi/httputil"
+	"github.com/matrix-org/dendrite/clientapi/jsonerror"
 	roomserverAPI "github.com/matrix-org/dendrite/roomserver/api"
 	"github.com/matrix-org/dendrite/userapi/api"
 	"github.com/matrix-org/dendrite/userapi/storage/accounts"
@@ -74,16 +76,32 @@ func JoinRoomByIDOrAlias(
 	}
 
 	// Ask the roomserver to perform the join.
-	rsAPI.PerformJoin(req.Context(), &joinReq, &joinRes)
-	if joinRes.Error != nil {
-		return joinRes.Error.JSONResponse()
-	}
+	done := make(chan util.JSONResponse, 1)
+	go func() {
+		defer close(done)
+		rsAPI.PerformJoin(req.Context(), &joinReq, &joinRes)
+		if joinRes.Error != nil {
+			done <- joinRes.Error.JSONResponse()
+		} else {
+			done <- util.JSONResponse{
+				Code: http.StatusOK,
+				// TODO: Put the response struct somewhere internal.
+				JSON: struct {
+					RoomID string `json:"room_id"`
+				}{joinRes.RoomID},
+			}
+		}
+	}()
 
-	return util.JSONResponse{
-		Code: http.StatusOK,
-		// TODO: Put the response struct somewhere internal.
-		JSON: struct {
-			RoomID string `json:"room_id"`
-		}{joinRes.RoomID},
+	// Wait either for the join to finish, or for us to hit a reasonable
+	// timeout, at which point we'll just return a 200 to placate clients.
+	select {
+	case <-time.After(time.Second * 20):
+		return util.JSONResponse{
+			Code: http.StatusAccepted,
+			JSON: jsonerror.Unknown("The room join will continue in the background."),
+		}
+	case result := <-done:
+		return result
 	}
 }