J.Lo definitely enjoyed her party, dancing and even getting behind the DJ booth at one point. Meanwhile, guests sipped summer-inspired cocktails by Ciroc Vodka and fought to snap their own selfie with the diva.
Earlier in the afternoon, the couple celebrated Lopez's big day with her kids -- 6-year-old twins Max and Emme with her ex-husband Marc Anthony -- on a boat in the Hamptons.
Lopez and Casper originally called it quits in June 2014 before rekindling their romance a few months later, kicking off this past New Year's together in Las Vegas.
The actress-singer-entrepreneur's actual birthday was on Friday, which she celebrated with a cheeky Instagram post of Marilyn Monroe.
Find out what Lopez had to say about Casper's boyfriend skills in the video below.