Katharine McPhee Files for Divorce ... From Husband She Cheated On

Katharine McPhee has finally pulled the plug on her marriage from the man she publicly cheated on.

Katherine's rep released a statement confirming the divorce, saying, "After six years of marriage, and having been separated for the past year, [Katharine] has filed for a dissolution of marriage from Nick Cokas."

The rep added, "The two have an amicable relationship and will remain friends."

"Amicable" is surprising ... considering 30-year-old McPhee humiliated 49-year-old Cokas last year by publicly making out with "Smash" director Michael Morris ... who also happens to be married.

As we reported, McPhee and Cokas tried reconciling earlier this Spring, but it just wasn't in the cards. We're told McPhee ultimately decided she just didn't want to be married anymore.