Weeks after the pair's split was revealed in June, Swift stepped out with actor Tom Hiddleston, and the new couple has been traveling around the world in the last few weeks.
"I figure if you're happy in your new relationship you should focus on that instead of trying to tear your ex bf down for something to do," Harris continued on Twitter. "I know you're off tour and you need someone new to try and bury like Katy ETC but I'm not that guy, sorry. I won't allow it."
Swift has long been rumored to be in a feud with fellow pop star Katy Perry, and her song "Bad Blood" is reportedly about the "Roar" singer. (Neither star has officially commented on the rumors).
"Please focus on the positive aspects of YOUR life because you've earned a great one. God bless everyone have a beautiful day," Harris wrote.