The actor, 41, whose sexual orientation has been the subject of speculation in the past, came out in a letter he sent on Wednesday to Russia's St. Petersburg International Film Festival, E! reports.
Wentworth's letter, which is posted on GLAAD's website, thanks festival organizers for an invitation but states that "as a gay man, I must decline. I am deeply troubled by the current attitude toward and treatment of gay men and women by the Russian government."
"Wentworth's bold show of support sends a powerful message to LGBT Russians, who are facing extreme violence and persecution: you are not alone," said a statement from GLAAD representative Wilson Cruz, who is also an actor (My So-Called Life).
Miller's announcement comes amid international condemnation of Russia's recent crackdown on homosexuals. Stringent new laws prescribe fines for people accused of spreading "propaganda of non-traditional sexual relations" to minors. Gay pride rallies are banned, as is the adoption of Russian-born children by same-sex couples. And foreign visitors suspected of being gay can now be detained for up to 15 days, which has raised concerns worldwide as Russia prepares to host the 2014 Winter Olympics.