The 37-year-old actress revealed she locked lips with the 'Safe House' star during a recent appearance on talk show 'Chelsea Lately'.
The smooch happened when they were filming a 'Sabrina The Teenage Witch' movie almost 20 years ago, but Melissa has revealed her confession wouldn't have shocked Blake - who has been married to Ryan since September 2012.
Speaking to People magazine at an event to launch Soleil Moon Frye's new guide book 'Let's Get This Party Started', in Beverly Hills, she said: ''It came out that Blake didn't know that Ryan and I had a make-out session when I was 19. She did know, actually. She reached out to me via email to help her with a birthday present for Ryan!''
More details of her kiss with Blake and her life in Hollywood from child star to adult actress will be revealed in Melissa's memoir 'Melissa Explains It All: Tales from My Abnormally Normal Life'.
The blonde star decided to lock lips with - who was crowned People's Sexiest Man Alive in 2010 - after he presented her with an expensive Beluga watch as a gift.
Melissa recalled to Chelsea Handler: ''He was 17 and I was, like, 19. I was like, 'You're too young for me.' So I stopped his car ... and I wasn't sure; am I gonna get mad at him?
''I wasn't sure so I just grabbed him and started kissing him. You get a piece of jewellery like that, you gotta make out with the guy!''
Melissa - who has three sons, Mason, seven, Brady, five, and 13-month-old Tucker with her husband Mark Wilkerson - also reveals she experimented with cannabis, ecstasy, magic mushrooms and mescaline.
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