The Audi Polo Challenge is held each year and this year was in aid of charities SkillForce and The Royal Marsden Cancer Charity, of which William is patron and president respectively.
Kate attended last year's match, along with the couple's puppy Lupo, to support William and Harry, but there was no sign of her today.
She is thought to be at her parents' home in Bucklebury, Berkshire, with Prince George.
On Friday Kensington Palace announced that the birth of the third-in-line to the throne had been formally registered after a registrar from Westminster City Council visited the palace.
William, 31, signed the birth register entry for his son, listing his and wife Kate's jobs as prince and princess of the United Kingdom.
The RAF search and rescue pilot is currently on paternity leave but is expected to return to work at some point next week.
His next official public engagement will be to visit the Anglesey Show later this month.
An announcement on his future plans is expected within the next fee months.