The former Spice Girl - who is currently a judge on 'The X Factor' Australia and also made guest appearances on the UK version of the series - said she attacked the music mogul after he made jibes about her weight while she was pregnant.
She told The Sun newspaper: ''I kept saying to him, 'Excuse me, I'm pregnant - do not be like that with me' and then at the end of the meal he called me fat. So I just slapped him.''
The singer admitted she was furious with Simon before she hit him because he was teasing her for ordering too much food and eating so much while they were dining out in a restaurant.
The feisty 37-year-old is good friends with Simon as well as former 'X Factor' judge Sharon Osbourne.
She recently said she would rather have a night out with Sharon than her daughter Kelly.
She said: ''I've had a night out with both Kelly and Sharon Osbourne before and I'd have to say I'd rather party with Sharon,'' Mel revealed to Australian radio show hosts Kyle and Jackie O.
''Kelly is adorable but Sharon is hilarious. She's crazy, she just doesn't care. She says it how it is, but is kind of respectful at the same time, if that makes sense?''