The insult was posted on Mr Musk's Twitter feed after he was alerted to the fact that Mr Gates had opted to buy Porsche's battery-powered Taycan sports car.
The Tesla boss then suggested he had been disappointed in discussions with Bill Gates.
"My conversations with Gates have been underwhelming tbh," he wrote in response to a Twitter user who alerted him to Mr Gates's decision.
Bill Gates admitted to having bought the Porsche during a video interview with YouTube star Marques Brownlee, in which the pair discussed electric vehicles.
Mr Gates actually began by lavishing praise on Tesla, saying that "if you had to name one company that’s helped drive [an effort to lower emissions], it’s them".
But he said that he had in fact bought a "very, very cool" Porsche Taycan for himself, eschewing Tesla's vehicles.
And he also suggested that there remained problems with electric vehicles, such as the high price and the problem of "range anxiety", which leaves drivers concerned their battery might run out with no way of quickly charging them up.
My conversations with Gates have been underwhelming tbh— Elon Musk (@elonmusk)February 18, 2020
In a follow-up tweet, Mr Musk was forced to admit that Bill Gates was at least impressive in one respect: his ability to jump over a chair.
True, that was pretty impressive— Elon Musk (@elonmusk)February 18, 2020
Mr Musk's remarks about Mr Gates are far from the first time he has been rude about fellow tech billionaires. He has in the past attacked Amazon founder Jeff Bezos for allegedly copying a plan to create an internet satellite, and Facebook boss Mark Zuckerberg for his perceived failure to understand the danger posed by artificial intelligence.