The couple had their first child Prince George - third in line to the British throne - on July 22.
Prince William, 31, took a brief break from parental duties as he played in a charity polo match in Ascot, Berks. It was his first public appearance since leaving hospital with Kate and their son.
Prince William joked that it was "good to get out of the house" - but said he had to buy some nappies for his son on his way home.
The prince said his son "likes to move around a lot, he wriggles a lot, he keeps us on our toes".
Prince William is currently on paternity leave from his job as an RAF search and rescue pilot.