The 61-year-old composer is undergoing treatment for the disease after doctors managed to catch it early, and has promised fans he'll be back to work before the New Year.
A statement from the star's representative reads: "Andrew Lloyd Webber has been diagnosed with prostate cancer. The condition is in its very early stages.
"Andrew is now undergoing treatment and expects to be fully back at work before the end of the year."
Webber is preparing to premiere Love Never Dies, a sequel to his hit Phantom of The Opera musical, in London's West End in March.