William was on the opposite team to his brother, Prince Harry, at the event held at musical impresario Andrew Lloyd Webber's Berkshire estate.
The Kent and Curwen Royal Charity Polo Cup was held in aid of Sentebale and the Tusk Trust charities, which the Princes support.
Harry's team won the game with a last-minute goal, and he was awarded the most valuable player award.
The Duke of Cambridge is due to compete in a second polo match today.
Meanwhile bookmakers Coral said the odds on Kate giving birth today had shortened from 5/1 to 4/1 after a flurry of bets.