As the singing competition was enjoying 'disco week', the show's presenter revealed Simon's own groovy look from back in the day by sharing a photo of the 57-year-old jokily flexing his muscles and posing for the camera in a vest top and braces with long Seventies-style hair while Sinitta smiles next to him.
Dermot shared the hilarious snap of Simon
While his co-judges burst into laughter over the old school snap, Simon went red with embarrassment, joking: "By the way, that was not me!"
Dermot then compared the judge to the show's most controversial contestant, Honey G, saying "Looking a little Honey G there aren't we?" before telling the crowd "I say Simon, you say Cowell!"
Dermot then joked that they should probably stop teasing him, saying: "He's probably about to sack me again," referring to his brief departure from the show last year.
The presenter has previously opened up about briefly leaving the show.
Dermot said he and Simon are "closer than ever" after briefly leaving the show last year
"I don't have a God-given right to that job," Dermot explained. "But I always said, 'If I find out they are talking to anyone else, I'm not going to play games, I'll just go'.
"So I called my wife and woke her up to tell her I was leaving The X Factor - which was a shock, as you can imagine…I called ITV and actually didn't speak to Simon. I texted Simon and got a lovely text back but at the time I wanted to keep that at arm's length."
Dermot admitted that Simon called him up to invite him back, revealing that he had said: "I've back a mistake, I'd love to get you back.
"Actually I think we're closer now than we ever have been. We text all the time."