"The fact is, I'm gay, always have been, always will be, and I couldn’t be any more happy, comfortable with myself, and proud," he wrote.
Cooper's sexuality has long been something of an open secret, but he has never publicly confirmed it. Even after gay magazine Out slapped him on its cover and said he was living in a "glass closet," he kept mum.
Cooper told Sullivan that he has always been out to his friends, family and colleagues. He said that, though he had wanted to keep a close hold on his private life, he had decided it was important to be more open with the world.
"I’ve begun to consider whether the unintended outcomes of maintaining my privacy outweigh personal and professional principle," he wrote. "It’s become clear to me that by remaining silent on certain aspects of my personal life for so long, I have given some the mistaken impression that I am trying to hide something - something that makes me uncomfortable, ashamed or even afraid. This is distressing because it is simply not true."
Cooper becomes at least the fifth openly gay cable news anchor. He joins his CNN colleague Don Lemon, along with HLN's Jane Velez-Mitchell and MSNBC's Rachel Maddow and Thomas Roberts.