Tito Ortiz Nude -- It was the work of a hacker!

Someone posted a full-frontal naked photo of MMA fighter Tito Ortiz through his Twitter page moments ago ... but Tito insists it was the work of a hacker.

The photo surfaced just a few hours ago -- when it was uploaded to Tito's personal photo-sharing website ... which is connected through his Twitter page.

The photo appears to be a naked self-portrait shot with a cell phone.

Moments ago, Tito tweeted, "Someone hack my f**king phone."