The 5ft 9in music mogul is already said to wear Cuban heels to make him look taller.
Simon was apparently spotted by members of the audience sitting on the cushion while filming auditions in Glasgow for the next instalment of the ITV show, the Daily Mirror reported.
He said: "He's a small man - but with a big opinion and a big personality. It's Simon to a tee... I took pictures and everyone was having a laugh about it."
A female fan who also noticed the chair added: "I think he's got the booster seat not because of David but because he wants to look bigger than the girls... When we spotted Simon's chair we were in stitches and lots of people were talking about it."
A spokesman for the star denied he used a booster seat, saying: "It's not to make him look taller, it's just a cushion he uses from time to time."