The two couples enjoyed a three-hour meal at Lucky's Steakhouse in Montecito, where the royal couple recently bought a $14million house.
WATCH Prince Harry and Meghan's rare joint appearance
It's not surprising that Meghan and Katharine are friends as the duo did musicals together when they were young.
Sharing a throwback of the two together last year, Katharine said: "Meghan and I did musicals together as kids. She grew up to be the Duchess of Sussex and I grew up to star on the West End, so same life if you ask me. #TBT."
Katharine and Meghan have known each other for years
Later that year, they reunited again at WE Day and talked about Archie, with whom she was pregnant at the time.
"She was lovely, and she remembered me – she remembered my sister more specifically because they were in the same class," the 36-year-old singer told the Evening Standard at the time. "And she said, 'I'm just sitting around waiting for the baby to come.'"
Katharine is pregnant with her first child
And earlier this year, Katharine spoke about her husband's relationship with the Prince, revealing they were "like father and son".
In an interview with Access Hollywood, she said: "My husband has a really, really beautiful relationship with Harry. They're like, they're so cute. They're like father and son."
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