The Canadian actress announced that she was a lesbian at a human rights conference for teachers and counsellors who work with young gay people.
She said she was "tired of lying by omission," and that her fear of coming out had caused her spirit, her mental health and her relationships to suffer.
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The audience applauded as she said: "I'm here today because I'm gay, and maybe I can make a difference to help others have an easier and more hopeful time."
Ellen later told photographers that she was "very happy" about her decision to speak out, but is currently single.
One teacher who was attending the conference told People that the atmosphere in the room was electric.
"It was exhilarating," said Mikki Cichocki. "You didn't know this was going to happen. You could see her sense of relief, like, 'I got it out. I said it out loud'."
Mikki went on: "There was this collective wave of emotion just supporting her, and a roar when she did actually say the words.
"You knew you just saw something that was momentous."
Ellen with Emile Hirsch who was once rumoured to be her boyfriend
Emile Hirsch, Lance Bass and Kristen Bell have all tweeted their support for Ellen since her statement.
The diminutive actress rose to fame after playing a pregnant teen in Indie movie Juno, for which she was Oscar nominated.