According to the Daily Telegraph, his brother, Prince Harry, threw the weekend bash at a country estate in eastern England.
About 20 friends were in attendance.
The newspaper says a nightclub owner who is a friend of the princes helped to organize the event.
Palace officials confirmed Monday only that the stag party had taken place.
"It has happened," a spokeswoman said, speaking on condition of anonymity in line with palace policy. "We are not giving any details."
Prince William announced his engagement to 29-year-old Kate Middleton in November. The pair will wed April 29 at London's Westminster Abbey.
The Associated Press contributed to this report.