"I am tired of hiding and I am tired of lying by omission," the 26-year-old actress said in a speech that drew roaring support from an audience at the Time to Thrive conference sponsored by the Human Rights Campaign.
"I'm here today because I am gay. And because ... maybe I can make a difference," she said. "To help others have an easier and more hopeful time. Regardless, for me, I feel a personal obligation and a social responsibility."
Page, who in 2007 played a pregnant teen in "Juno," has also starred in "Inception", "To Rome with Love" and "X-Men: The Last Stand."
Page is also working on "Freeheld," a drama about a terminally ill police detective fighting to assign her retirement benefits to a lesbian lover.
Page said in her speech that she suffered for years because she was scared to be public about her sexuality.
"My spirit suffered, my mental health suffered and my relationships suffered," she said. "And I'm standing here today, with all of you, on the other side of all that pain."
The Human Rights Campaign, a lesbian, gay, bisexual and transgender advocacy group that posted her speech on its website, congratulated Page for completing the "deeply personal and arduous journey" of coming out.
Page in her speech also praised others who have taken similar steps, such as athlete Michael Sam, who announced he was gay this month and could become the National Football League's first openly homosexual player.