The Star Trek actor said that the suicide of gay teenager Jamey Rodemeyer was the catalyst for his decision to be open about his sexuality.
"It became clear to me in an instant that living a gay life without publicly acknowledging it is simply not enough to make any significant contribution to the immense work that lies ahead on the road to complete equality," Quinto wrote on his official blog.
"Our society needs to recognise the unstoppable momentum toward unequivocal civil equality for every gay, lesbian, bisexual and transgendered citizen of this country.
"Gay kids need to stop killing themselves because they are made to feel worthless by cruel and relentless bullying," Quinto continued. "Parents need to teach their children principles of respect and acceptance.
"Jamey Rodemeyer's life changed mine, and while his death only makes me wish that I had done this sooner, I am eternally grateful to him for being the catalyst for change within me," the actor added. "Now I can only hope to serve as the same catalyst for even one other person in this world."
Quinto officially came out in an interview with New York magazine, in which he discusses his decision to star in Angels in America.
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Watch Zachary Quinto in the trailer for Star Trek below: