According to People and other sources, the "World War Z" star was seen arriving at the party, held at the Sunset Tower Hotel in Los Angeles on Saturday, Feb. 9.
"For well over a decade now they have lived totally separate lives and so not surprisingly, they have become totally different people than who they once were when they were together", said the source.
"Brad, for long, was a very important part of Jen's life".
However, she "didn't really pay any special attention to Brad" as she was also "busy making sure all of her other guests had an awesome time". "They hugged and chatted for a bit, but Jen was busy making sure all of her other guests had an fantastic time".
Among the big names were Ellen DeGeneres, George and Amal Clooney, Orlando Bloom and Katy Perry, along with Reese Witherspoon, who partied until she literally fell down.
She may have lived for half a century on this planet but Jennifer looks nothing like her age.
"Love these ladies", Hudson wrote. "Celebrated this beauty last night Happy Bday JA!"
Her star-studded birthday party was given a strict social media ban.
"Our ray of sunshine, example of perpetual goodness". The former couple split in February 2018 after three years of marriage.
Speaking to The New York Times, Theroux revealed that it was "the most gentle separation, in that there was no animosity".
"I don't have time to get her a present". I really don't. My marriages, they've been very successful, in [my] personal opinion. And when they came to an end, it was a choice that was made because we chose to be happy, and sometimes happiness doesn't exist within that arrangement anymore.
The medium said there were no reports of Pitt and Aniston mingling throughout the evening, though.
Then Pitt met Angelina Jolie on the set of Mr & Mrs Smith in 2003, ditched Aniston, had six kids and decamped to a French chateau with the actor-turned-humanitarian.