Ashton Kutcher and Mila Kunis' first kiss was caught on camera - when she was just 14.

The actor reveals the pair's smooch when he and Mila were co-stars on TV comedy That 70s Show was the first time she had locked lips with a guy.

In a new interview with satellite radio host Howard Stern Show, the 39-year-old actor said, "I was 19, she was, like, 14... I think I was her first kiss, like, on the show. We have our first kiss memorialised on a TV show!"

Back then, Ashton used to help the teen actress with her chemistry homework, and he admits it was weird kissing a girl who was like his "little sister" on the show: "I was like, 'Isn't this illegal?'" he laughed, "and it was really awkward, because I'm, like, a 19-year-old kid. She was 14! She was, like, my little sister. I wanted to make sure she was OK."

But when Stern suggested Mila, who is now Kutcher's wife, wasn't always a fan of his looks, Ashton protested, pointing out, "She has, like, a journal or, like, a diary from when she was a kid where she wrote, 'Oh, this guy's hot!' She thought I was cute!

"At first, I think she thought that I was good looking, and then shortly thereafter, I was just annoying to her, because I was like a big brother. It was like, 'He's annoying, because he's always looking out for my best interest'."

The former co-stars hooked up after his marriage to Demi Moore fell apart, and they wed on 4 July, 2015. The pair recently its second child, son Dimitri.