Speaking to the Observer Magazine, Sheffield-born 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, who obtained a degree in chemistry at University of Sheffield before gaining a PhD from Birbeck, University of London, made history when she participated in a mission to the Soviet modular space station Mir in May 1991.
Flood relief funding and need for joined-up action before and after devastation – The Yorkshire Post saysBrexit must not put United Kingdom at risk – Dan JarvisIn 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."