What happens when the creative director and designer for sexy brand Balmain, Olivier Rousteing, has a birthday party? The hottest celebrities show up in their tightest and most revealing outfits ready to celebrate. In a sea of Kardashians and Jenners — Kim, Kourtney, Kris, Kendall, and Kylie were all there [1] — we spotted Jennifer Lopez [2] arm in arm with (let's just say it, boyfriend) Casper Smart.

In a long-sleeved top with cutouts perfectly placed to show off her toned abs, a high-waisted pleated skirt, and black, blue, and gold accessories, J Lo looked as sexy as ever as she spent the night posing for a selfie with Kylie (whose boyfriend, Tyga, was performing) and partying the hours away with the Balmain army. Keep scrolling to see more photos of Jennifer and Casper at the gathering, then take a look back at their relationship through the years [3].

Jennifer Lopez [4] and Kylie Jenner

Jennifer Lopez [5] and Olivier Rousteing

Casper Smart and Jennifer Lopez [6]

Jennifer Lopez [7]

Casper Smart, Jennifer Lopez [8], and Olivier Rousteing

Casper Smart and Jennifer Lopez [9]