The 30-year-old singer, who visited the Ali Forney Center in Brooklyn as part of the Share Kindness campaign, said she has never talked about it with anyone before, reported The Huffington Post.
"I suffer from a mental illness- I suffer from PTSD. I've never told anyone that before, so here we are," the singer said.
Gaga went on to say that "kindness" has helped her fight the illness.
"But the kindness that's been shown to me by doctors - as well as my family and my friends - it's really saved my life. I've been searching for ways to heal myself, and I've found that kindness is the best way," she said.
"These children are not just homeless or in need. Many of them are trauma survivors. They've been rejected in some type of way.
"I am no better than any of those kids. And I'm no worse than any of them. We are equal. We both walk our two feet on the same earth. And we're in this together," she said.
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