Helen Sharman, who visited the Soviet Mir space station in 1991, told the Observer newspaper on Sunday that "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 went on. "Will they be like you and me, made up of carbon and nitrogen? Maybe not."
Then, in a tantalizing theory that should probably make you very suspicious of your colleagues, Sharman added: "It's possible they're here right now and we simply can't see them."
Sharman was the first of seven Britons to enter space.
The chemist spent eight days as a researcher on the space mission when she was 27, making her one of the youngest people to enter orbit.
NASA rovers are trawling Mars for evidence of past or present life forms, but humankind's endless fascination with extraterrestrial life forms has so far proved fruitless.
Sharman is not the only person to speculate that we've had brushes with aliens, though.
Sharman also discussed her frustration with observers defining her by her sex. "People often describe me as the first British woman in space, but I was actually the first British person. 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."