The American fashion model, 24, looked picture perfect as she posed for a selfie with the pretty British royal, 28, in front of a beautiful water-view backdrop outside the grand Hôtel du Cap-Eden-Roc.
Taking to her Instagram page on Sunday, Karlie captioned the photo of the pair smiling with the simple words, "My wedding date".
The ladies were guests at the wedding ceremony of art collector and dealer Alberto "Tico" Mugrabi and Colby Jordan.
Just moments before, Karlie wished the newlyweds the very best for their future together, by writing, "an unforgettable night celebrating a beautiful couple. Wishing you a lifetime of happiness Tico and Colby."
Although many would be surprised to find out that Karlie and Princess Beatrice are good friends, the duo have been pictured hanging out together once before.
Earlier this month, they were seen sharing the same box at the U.S. Open Tennis Championships – along with other notable faces, including Ivanka Trump.
Meanwhile, Karlie and Beatrice's girls' day out comes weeks after the young royal ended her ten-year relationship from boyfriend Dave Clark.
At the time sources told HELLO! that the pair decided to call it quits after they reportedly had a serious discussion about their future together.
A source close to the York family said: "They broke up a month ago. It was all mutual and very amicable. After ten years together, they needed to work out what they both wanted and have some space."
They continued: "It could be that once they have had some time apart, they might decide to get back together. It's very clear they have the support of the Duchess and the whole family and they (the York family) are very open minded about what might happen in the long term. The Duchess is very fond of Dave."