Dr Leroy Chiao made the admission when he visited the University of Wollongong in New South Wales, Australia, to speak to students.
As ISS commander, Dr Chiao spent months in space dwelling on whether aliens are out there.
Recent discoveries of water on one of Saturn’s moons, and amino acids on comets, left him in no doubt about extra-terrestrial life existing, he said.
He added: “To find this kind of evidence of life in our own backyard means to me the idea there’s other life in the universe is pretty much a given.
“I think there’s all kinds of life out there, including intelligent life, but the reason we haven’t found each other is because of vast distances.”
He also spoke about the experience of being in space.
He said: “It’s very physical. You’re working against the pressures of the suit and you’re aware of where you’re putting your tethers.
“It’s surreal and like a dream. You can’t believe you’re out there in a suit doing this kind of work.
“But at the same time you’re well trained, but if you rest for a minute and look where you are, you think ‘Is this real?'”
The ISS orbits Earth every 90 minutes and the crew sees 16 sunrises and sunsets per day
He added: “I don’t think anyone flies into space without some form of fundamental introspection about what life is about.
“For me, I was surprised how much more beautiful it is than what the pictures show.
“It looks very peaceful and nice, and intellectually I knew there was war, famine and conflict down there and that dichotomy was hard to reconcile.
“It makes you stop and think, and it gave me that perspective of what’s important in life.”