The L.A. Ink star (whose real name is Katherine von Drachenberg) tweeted her relationship with the Canadian techno music star (whose real name is Joel Thomas Zimmerman) is "indeed over," implying he might not have been the most faithful beau.
"Man, I'm a fool. Cheating on your loved one is the most hurtful thing one could do," she wrote. "I hate to have to admit, that this relationship is indeed over."
The celebrity couple had been dating since last September, and got engaged in December.
Von D was previously linked to Sandra Bullock's ex Jesse James.
UPDATE 12:06 p.m. -- DeadMau5 has taken to his personal Facebook page to deny that he cheated.
“Kat and I split in November and I moved back home to Toronto," he wrote. "During this time, while we were broken up, I did have relations with another woman. Following that, Kat and I reconciled and I acknowledged being intimate with another person while we were estranged.
"We did move past those issues and I proposed to Kat in mid-December. At the end of June, it was clear that the relationship was not working and we mutually ended the engagement. I was not, at any point, unfaithful to Kat during our time together."
The 32-year-old added, “I don’t wish to go tit for tat with these kinds of pronouncements and don’t expect to comment further, but I do believe that those who have expressed concern deserve a more complete understanding of what transpired."