The 60-year-old music mogul was taken to hospital for an operation, his spokeswoman said.
"He's doing fine, he's under observation and is in the best possible hands," she said after he was admitted.
Cowell, best known for hit shows The X Factor and Britain's Got Talent, is spending lockdown in the US, where he now lives.
"Simon has broken his back and is having surgery this evening," his spokeswoman confirmed, soon after his arrival at hospital. The surgery began on Saturday evening Los Angeles time.