diff --git a/net/rpmsg/rpmsg_sockif.c b/net/rpmsg/rpmsg_sockif.c
index 5ce5021ff1d..e1d547f4f83 100644
--- a/net/rpmsg/rpmsg_sockif.c
+++ b/net/rpmsg/rpmsg_sockif.c
@@ -296,9 +296,11 @@ static int rpmsg_socket_ept_cb(FAR struct rpmsg_endpoint *ept,
 
   if (head->cmd == RPMSG_SOCKET_CMD_SYNC)
     {
+      rpmsg_socket_lock(&conn->recvlock);
       conn->sendsize = head->size;
       conn->psock->s_flags |= _SF_CONNECTED;
       _SO_SETERRNO(conn->psock, OK);
+      rpmsg_socket_unlock(&conn->recvlock);
       rpmsg_socket_post(&conn->sendsem);
       rpmsg_socket_pollnotify(conn, POLLOUT);
     }