Brad, 55, split from Jennifer in 2005 after five years of marriage.
Despite various reports about whether Pitt and Aniston are still friends, his appearance at her party put the rumours to bed once and for all.
Aniston also apparently didn't spend a lot of time with him, opting to hang out with her gal pals Gwyneth Paltrow and Kate Hudson instead.
"Happy Birthday to this fierce Woman", he captioned a photo of Jen holding some horns over her head. We remember when Pitt and Aniston went on their first date all the way back in 1998, and sparks flew from there.
Another source told the Entertainment Tonight website that when Pitt left the party, the hotel staff and his security team helped ensure that it was hard to take photographs of him. Mayer's ex, Katy Perry, attended the party too - with boyfriend Orlando Bloom.
"Jen has always been a hard worker and a perfectionist when it comes to filming", a source previously told PEOPLE. "They are friendly, but that's it", the grapevine said.
Opening up about her two divorces-the actress was also married to Justin Theroux from 2015 until 2018-Aniston told ELLE, "I don't feel a void".
After dating for nearly a decade and with six children together, Pitt and Jolie married in 2014, only to file for divorce two years later, after Jolie accused Pitt of heavy drinking and an altercation with their son Maddox, 17.
The couple said in the statement they made the announcement to avoid tabloid speculation. "This decision was mutual and lovingly made at the end of a year ago". "They are friends and have supported each other through some tough times".
Many wondered why Pitt would show up to Aniston's party at all if it wasn't to rekindle a romance.
When it comes to celebrity love triangles, there are few more famous than Brad Pitt, Jennifer Aniston, and Angelina Jolie. They married in 2014, after spending over a decade together. The exes are now engaged in a tumultuous custody battle.