The X Factor supremo, 60, had a fall from a new electric bike he was testing at his house in Malibu.
Cowell, who was with his family at the time, hurt his back and was taken to hospital, a spokeswoman said.
“Simon has broken his back and is having surgery this evening,” she confirmed.
The spokeswoman had earlier said: “He’s doing fine, he’s under observation and is in the best possible hands.”
It is understood the surgery started on Saturday evening Los Angeles time.
Entertainment mogul Cowell has a six-year-old son Eric with his partner Lauren Silverman.
Just last month it emerged that Cowell has bought Sony Music Entertainment’s stake in TV programmes including Britain’s Got Talent and The X Factor.
The move sees the music mogul gain ownership of all the international versions of The X Factor and Got Talent programmes.
The Got Talent format airs in 76 markets, while The X Factor is broadcast in more than 130 territories.
Sony Music’s share of the venture has transferred to Syco Entertainment, a company solely owned by Cowell.
Filming on the next stage of this year’s Britain’s Got Talent has been postponed because of the coronavirus crisis but ITV has said it hopes to continue the series later this year.
