A few of Aniston's friends gave a glimpse inside the party, thanks to a fun photo booth set up inside. Gwyneth Paltrow -- who interestingly enough has her own history with Pitt, who's her ex-fiance -- shared her snaps on Instagram on Sunday.
"Happy birthday (last night and tomorrow) to #jenniferaniston our ray of sunshine, example of perpetual goodness," Paltrow wrote. "We love you so much and you make 50 LOOK [fire emoji]."
Aniston was surrounded by her close gal-pals, including jewelry designer Jennifer Meyer, who shared snaps with Aniston surrounded by friends including Sara Foster and Jason Bateman's wife, Amanda Anka.
"Last night celebrating our beautiful Jen ❤️❤️," she wrote. "WE ALL LOVE YOU SO MUCH."
Makeup artist Gucci Westman was also in attendance -- and Instagrammed a photo booth pic that included Witherspoon -- writing, "Celebrating one of my nearest and dearest last night!! How exciting does Jen make it look to get older ?! 💕❤️."
An eyewitness tells ET that Pitt arrived at his ex-wife's birthday party with his security team. According to the eyewitness, he arrived 45 minutes after Aniston and seemed to be in a "good mood and happy to be there."
When ET spoke to Aniston back in December, she said DeGeneres was actually going to throw her a birthday party. Watch the video below for more: