The rock legend said his wife Penny had been upset at how the host and judge acted towards her when she was on the celebrity version of the hit BBC show in 2021.
He spoke as an investigation was launched into complaints about Wallace over a 17-year period.
And a letter signed by 13 people this week claimed they had witnessed his behaviour, including former BBC Newsnight host Kirsty Wark.
Wallace reportedly made sex-related jokes, thrust his body towards a woman and wandered around the studio "completely naked except for [a] sock pulled over his penis".
Sir Rod, 79, blasted the 60-year-old.
He wrote: "You humiliated my wife when she was on the show, but you had that bit cut out, didn't you? "You're a tubby, bald-headed, ill-mannered bully. Karma got ya." Kirsty, who was on Celebrity MasterChef in 2011, claimed Wallace told jokes of a "sexualised nature" in front of contestants and crew she felt were "really, really in the wrong place.
She told how her experience on Celebrity MasterChef was "joyous", but added: "The fly in the ointment, on occasion, was Gregg Wallace.
"I think people were uncomfortable.
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