The X Factor judge and music mogul, 60, was expected to have surgery for the injury on Saturday night, according to a spokesperson.
The spokesperson said: "Simon had a fall from his bike on Saturday afternoon whilst testing his new electric bike in the courtyard at his house in Malibu with his family."
She added: "Simon has broken his back and is having surgery this evening. He’s doing fine, he’s under observation and is in the best possible hands."
It is understood the surgery started in the evening Los Angeles time.
Mr Cowell has reportedly spent coronavirus lockdown in Malibu with his partner Lauren Silverman and young son Eric, as well as her teenage son from a previous relationship.
He told People in May: "We definitely have more family time together now. We watch movies in the evening, play together, read books, comics, everything."
Mr Cowell also founded the Got Talent series, which includes Britain's Got Talent and America's Got Talent, among others.
Last month it emerged that Mr Cowell had bought up Sony Music Entertainment’s stake in several TV programmes - including Britain’s Got Talent and The X Factor.
The move sees the him 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.
Filming on the next stage of this year’s Britain’s Got Talent has been postponed because of coronavirus but ITV has said it hopes to continue the series later this year.