The former Prison Break star, 41, went public with the news in a letter to the St. Petersburg International Film Festival director Maria Averbakh, who had invited him to attend.
According to the message, which was posted on GLAAD's website Wednesday, Miller thanked Averbakh for the "kind invitation" but turned it down because, "As a gay man … I am deeply troubled by the current attitude toward and treatment of gay men and women by the Russian government."
Added the actor: "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. … If circumstances improve, I'll be free to make a different choice."
Miller's words refer to recent anti-LGBT legislation passed by the country. Signed by President Vladimir Putin in June, the law bans the "propaganda of nontraditional sexual relations" and imposes hefty fines for providing information about the gay community to minors or holding gay pride rallies, the Associated Press reports.
There also is concern about a long-running problem of violence against gays in Russia, as well as a new law restricting adoptions of Russian children by people in countries allowing same-sex marriage.
E! News was first to report the posting of Miller's letter.