It was the second day in a row Markle showed up to support her prince.
On Saturday, they attended a public event as a couple for the first time, a significant step in their relationship.
"Their royal highnesses have taken part in polo fixtures in support of their charities since 2007 and have continued each year since, raising over 10 million pounds [$13 million] for charitable causes to date," Kensington Palace said in a statement Saturday.
A slew of A-list celebrities — including Eddie Redmayne and his wife, Hannah Bagshawe; "The Crown" actor Matt Smith; and actor Tom Hardy — were on hand Saturday. Emma Stone made it for the event Sunday. The star studded polo match kicks off the social season in London every spring.
Markle has started to attend a number of high-profile private events with Harry. In March she accompanied him to a wedding in Jamaica for a childhood pal from Eton, the school he attended as a teenager.
Harry joined Markle, an actress on "Suits," over Easter instead of spending Easter at Windsor with his family, indicating just how serious the relationship has become.
Harry and Markle met last summer, when they were introduced by friends in common, and the two were photographed together for the first time last November attending a play and leaving the private Members Club Soho in February holding hands.
They have been nearly inseparable, with the two jetting back and forth across the Atlantic to spend time together. The couple took a romantic vacation together after the new year to view the Northern Lights in Norway.
Recently Markle has spent weeks at Harry's Nottingham Cottage, and speculation has been rife that an engagement is likely in the coming months. She recently shut down her lifestyle blog The Tig and gave up her fashion line at Canadian department store Reitmans — two steps that are necessary if she is to join the royal family.
Kensington Palace announced that Harry will be making an overseas tour in June to Singapore and then to Sydney, which will host the 2018 Invictus Games, the international sporting competition for wounded, injured and sick servicemen and -women founded by the prince in 2014. This fall the Invictus Games will be held in Toronto, where Markle films "Suits."