"Prison Break" star Wentworth Miller has just revealed he's a gay man ... in a statement that reads as a giant "F YOU" to Russia.

The 41-year-old actor fired off a letter to the director of the St. Petersburg International Film Festival -- rejecting its invitation for him to travel to the Motherland for the event ... and in the letter, WM went public about his sexuality for the first time.

"Thank you for your kind invitation. As someone who has enjoyed visiting Russia in the past and can also claim a degree of Russian ancestry, it would make me happy to say yes. However, as a gay man, I must decline."

Miller continues ... "I am deeply troubled by the current attitude toward and treatment of gay men and women by the Russian government."

"The situation is in no way acceptable, and I cannot in good conscience participate in a celebratory occasion hosted by a country where people like myself are being systematically denied their basic right to live and love openly."

He added, "If circumstances improve, I'll be free to make a different choice."