It's something of a recreational diversion for the couple, who otherwise are spending the bulk of their three-day trip to Southern California promoting British business ties to the U.S.
The Duke and Duchess of Cambridge arrived by helicopter in the coastal city of Carpinteria in Santa Barbara County, joining hundreds of well-heeled royal watchers who had traveled long distances and paid big sums to see them.
Among the celebrities spotted at the event were Jennifer Love Hewitt, Zoe Saldana, Rosario Dawson, Billy Zane, Rob Lowe, model-actress Molly Sims and musician Joe Jonas.
William wore a white shirt and trousers with a blue blazer while the former Kate Middleton sported a silver and marble-grey, hand-painted silk dress by British designer Jenny Packham.
The cheap seats cost $400, while anyone paying $4,000 for a VIP ticket would get a chance to hobnob with the royal newlyweds. The duchess will present the trophy to the winning polo team.
Carolyn Sesvold flew in from Chicago to attend the match, buying a brown-and-white polka dotted dress, hat and gloves inspired by the outfit Julia Roberts wore to the polo match in the movie "Pretty Woman."
Sesvold, a royal fan, also came to watch a family friend who paid about $50,000 to play against William in the polo game. Those playing on the same team as the duke paid twice that, she said.
Cheryl Reyes, who works in an office in Orange County, bought her $400 ticket as soon as she heard the royal couple would be coming to Santa Barbara. Standing across the field nibbling on a sandwich and drinking a flute of juice, she worried about whether she'd be able to see the duchess.
"I wanted to see Princess Catherine," she said. "There's just something magical about being a commoner and ending with a prince. She's the one I want to see."
Both Prince William and his brother, Prince Harry, regularly play polo during the summer months. The horseback sport, sometimes referred to as the game of kings, is known for its fast pace, dangerous maneuvers and exclusivity. William plays with a one-goal handicap, which means he plays to a high standard.
The duke and duchess arrived in California on Friday after a nine-day trip to Canada, their first foreign excursion since getting married in April.
After the polo game, the duke and duchess will head to downtown Los Angeles to attend a black tie event at the historic Belasco Theatre, where they will be guests of honor at a British Academy of Film and Television Arts dinner honoring 42 young British filmmakers.
The list of celebrities reportedly also attending reads like a who's who of Hollywood royalty: Tom Hanks, Nicole Kidman, Quentin Tarantino and Judd Apatow.
William and Kate's arrival in California has been a lower-key affair compared to the largely rapturous welcomes they received as they crisscrossed Canada, though small crowds of royal watchers waving British and American flags congregated outside the British consul general's home in Hancock Park and elsewhere. Among the celebrities seen chatting at a reception for the couple were British football star David Beckham and humorist Stephen Fry.
On Sunday, the royal couple will watch a dance at a nonprofit academy in the gritty Skid Row area of downtown then attend a job fair for U.S. servicemen and women transitioning to civilian life.
"People just want a glimpse of the duke and duchess," said Los Angeles resident Christian Kang. "There's so much compassion in both of them. I know they're going to Skid Row, and I think that's very good to see from someone in his position."
Though Prince William has been to America before, it is Kate's first trip to the U.S. William's late mother, Princess Diana, who would have turned 50 this month, charmed Americans when she visited in the 1980s.