diff --git a/src/github.com/matrix-org/dendrite/clientapi/routing/register.go b/src/github.com/matrix-org/dendrite/clientapi/routing/register.go
index ef857770..d3797634 100644
--- a/src/github.com/matrix-org/dendrite/clientapi/routing/register.go
+++ b/src/github.com/matrix-org/dendrite/clientapi/routing/register.go
@@ -866,6 +866,7 @@ type availableResponse struct {
 // RegisterAvailable checks if the username is already taken or invalid.
 func RegisterAvailable(
 	req *http.Request,
+	cfg config.Dendrite,
 	accountDB *accounts.Database,
 ) util.JSONResponse {
 	username := req.URL.Query().Get("username")
@@ -877,6 +878,17 @@ func RegisterAvailable(
 		return *err
 	}
 
+	// Check if this username is reserved by an application service
+	userID := userutil.MakeUserID(username, cfg.Matrix.ServerName)
+	for _, appservice := range cfg.Derived.ApplicationServices {
+		if appservice.IsInterestedInUserID(userID) {
+			return util.JSONResponse{
+				Code: http.StatusBadRequest,
+				JSON: jsonerror.UserInUse("Desired user ID is reserved by an application service."),
+			}
+		}
+	}
+
 	availability, availabilityErr := accountDB.CheckAccountAvailability(req.Context(), username)
 	if availabilityErr != nil {
 		return util.JSONResponse{
@@ -887,7 +899,7 @@ func RegisterAvailable(
 	if !availability {
 		return util.JSONResponse{
 			Code: http.StatusBadRequest,
-			JSON: jsonerror.InvalidUsername("A different user ID has already been registered for this session"),
+			JSON: jsonerror.UserInUse("Desired User ID is already taken."),
 		}
 	}
 
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 ced000bb..6081fc4a 100644
--- a/src/github.com/matrix-org/dendrite/clientapi/routing/routing.go
+++ b/src/github.com/matrix-org/dendrite/clientapi/routing/routing.go
@@ -139,7 +139,7 @@ func Setup(
 	})).Methods(http.MethodPost, http.MethodOptions)
 
 	r0mux.Handle("/register/available", common.MakeExternalAPI("registerAvailable", func(req *http.Request) util.JSONResponse {
-		return RegisterAvailable(req, accountDB)
+		return RegisterAvailable(req, cfg, accountDB)
 	})).Methods(http.MethodGet, http.MethodOptions)
 
 	r0mux.Handle("/directory/room/{roomAlias}",