Well, they are celebs of the Instagram era, right?
RiRi was taking the chance to catch some rays in between shows on her Diamonds world tour, and snapped herself sipping on a cocktail in a blue bikini.
Meanwhile, 'Sports Illustrated' model Kate Upton sent temperatures soaring even more with her selfie, where she posed for (her own) camera in a frilly white bikini.
But which beach babe comes out on top? Well, dear readers, we'll leave that up to you...