They remained friends throughout his ill-fated relationship with Angelina Jolie.
And eagle-eyed onlookers spotted Brad Pitt, 55, sneaking past the paparazzi as he joined the stars at ex-wife Jennifer Aniston's 50th birthday party in Los Angeles on Saturday night.
Actor Brad, who's reportedly been quietly dating Charlize Theron since January, went incognito in a flat cap as he rushed inside Jennifer's favorite West Hollywood spot, the historic Sunset Tower.
Time to celebrate: Jennifer Aniston partied in style Saturday night as she celebrated her 50th birthday with an exclusive party
Still Friends: Brad Pitt, in flat cap, center, snuck into the star-studded bash, which was held at West Hollywood's Sunset Tower
He wasn't alone, with stars including Katy Perry and boyfriend Orlando Bloom, chat show host Ellen DeGeneres, Gwyneth Paltrow, Barbra Streisand, Jason Bateman and Kate Hudson joining the celebrations.
Jennifer, whose birthday falls on February 11, attended the soirée in eye-catching style, stepping out in a stylish all-black jumpsuit with criss-crossing string backing.
With her golden locks styled in voluminous waves and her makeup applied in typically impeccable style, the star looked relaxed and happy as she was feted by her famous friends, as well as her actor dad John Aniston.
Birthday couture: Jennifer showed up to her own party is a stylish all-black jumpsuit with criss-crossing string backing
As they were: The two are pictured in 2004 at Cannes, a year before they split
Tabloid stars: Brad and Jennifer quickly became the ‘it’ couple of the early 2000s, with their every move making headlines. Two years after they met, Aniston and Pitt married
Still friends: The two divorced in 2005, though they've managed to keep up a cordial relationship in the years since
Also among the attendees was George Clooney, who arrived with his glamorous wife Amal. Jennifer has long been friends with George and visited him and Amal at their lake house in Como last summer.
During an appearance on The Ellen DeGeneres Show, the Friends star revealed how incredible it was to see George become a father.
'It’s amazing to see him with children. That is what we talked about, that anything can happen,' she told the host.
After several years of being celebrated as one of Hollywood's most glamorous couples, Jennifer and Brad shocked fans when they announced that they were divorcing in 2005.
The two began dating after meeting on a blind date their respective agents set up in 1998. They quickly became the ‘it’ couple of the early 2000s, with their every move making headlines.
Two years after they met, Jennifer and Brad tied the knot during a romantic ceremony in Malibu, California. Their marriage ended on January 5, 2005. The couple announced that they had ‘formally’ separated.
It soon emerged that he had begun a relationship with Angelina Jolie, with the actress later confirming years later that they fell in love while filming Mr And Mrs Smith in 2003, while he was still married to Jennifer.
Throughout his 11-year relationship with Angelina, tabloid stories regularly emerged that Brad was still in contact with Jennifer, seeking advice from his former wife despite his new love's objections.
And it seems that following Brangelina's nasty divorce, Brad may well have renewed his friendship with his ex.
Need for speed: Katy Perry and her beau Orlando Bloom arrived at Jen's party on his gray Husqvarna motorcycle
Keeping it simple: The lovebirds were casually-dressed for their outing to the exclusive soiree
On Saturday, the exes had plenty of mutual friends to celebrate with. Katy Perry and Orlando Bloom turned heads as they arrived at Jen's party on his gray Husqvarna motorcycle.
Orlando wore jeans and a blue fleece-lined jacket, while Katy fittingly rocked a pair of shiny vinyl pants and an edgy leather biker jacket.
Chat show host Ellen DeGeneres was also spotted joining the birthday shindig, looking casually cool in tailored tracksuit bottoms with a navy bomber jacket.
During the Dumplin' star's appearance on The Ellen Show, the host promised a special hour-long appearance for her friend on the occasion of her 50th birthday, but that didn't stop her from celebrating a bit early.
Iron Man and Avengers co-star Robert Downey Jr. showed up to Jennifer's shindig looking every bit like Tony Stark.
Firm friends: Ellen DeGeneres was among the celebrity guests
The acclaimed actor made one of the toasts during Jennifer's ill-fated wedding to Justin Theroux, which ended with their separation in late 2017.
Jason Bateman, her co-star in 2010's The Switch, stepped out of a car with a goatee, a rarity for the usually clean-shaven actor.
Kate Hudson stopped by the birthday party in a shining black dress. The How To Lose A Guy In 10 Days actress recently appeared with Jennifer in the romantic comedy Mother's Day.
There was also plenty of Hollywood royalty at the Marley & Me star's bash.
Barbra Streisand and her husband James Brolin made an appearance.
Diane Keaton arrived in her trademark finger-less gloves and wide-brimmed black hat.
Superhero in the flesh: Iron Man and Avengers co-star Robert Downey Jr. showed up to Jennifer's shindig looking every bit like Tony Stark
Icons: There was also plenty of Hollywood royalty at the Marley & Me star's bash. Barbra Streisand and her husband James Brolin made an appearance
Unmissable: Diane Keaton arrived in her trademark finger-less gloves and wide-brimmed black hat
A smiling David Arquette, a figure from Jennifer's Friends days, made a special appearance.
After he married her co-star Courteney Cox, the NBC sitcom's cast famously appended '-Arquette' to all of their names to poke (loving) fun at their colleague.
Jennifer Meyer, who designed the Horrible Bosses star's wedding ring from her marriage to Justin Theroux, dropped by the party as well.
Going casual: Jason Bateman, her co-star in 2010's The Switch, stepped out of a car with a goatee, a rarity for the usually clean-shaven actor
Happy to be there: A smiling David Arquette, a figure from Jennifer's Friends days, made a special appearance