And they might be on Earth without us knowing, she said.
Speaking to the Observer Magazine, Dr Helen Sharman said: "Aliens exist, there's no two ways about it. There are so many billions of stars out there in the universe that there must be all sorts of different forms of life."
She added that although they may not be made up of carbon and nitrogen like humans, "it's possible they're here right now and we simply can't see them".
Dr Sharman, 56, made history when she participated in a mission to the Soviet modular space station Mir in May 1991.
"There’s no greater beauty than looking at the Earth from up high – and I’ll never forget the first time I saw it," she said.
"After take-off we left the atmosphere and suddenly light streamed in through the window. We were over the Pacific Ocean.
"The gloriously deep blue seas took my breath away."
In the interview, she highlighted how she is often referred to as the first British woman in space, rather than simply the first Briton.
"It's telling that we would otherwise assume it was a man," she said.
"When Tim Peake went into space, some people simply forgot about me. A man going first would be the norm, so I'm thrilled that I got to upset that order."
Additional reporting by agencies
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