Brad Pitt makes an appearance at ex-wife Jennifer Aniston’s 50th birthday celebration

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.

Aging gracefully: Jennifer Aniston partied in style Saturday night as she celebrated her upcoming 50th birthday

Still Friends: Brad Pitt, in flat cap, center, snuck into the star-studded bash, which was held at West Hollywood's Sunset Tower

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’s actual birthday is still two days away, but she was still game to arrange a soiree.

Jennifer showed up to her own party is a stylish all-black jumpsuit with criss-crossing string backing. 

Birthday couture: Jennifer showed up to her own party is a stylish all-black jumpsuit with criss-crossing string backing

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

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

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

Still friends: The two divorced in 2005, though they’ve managed to keep up a cordial relationship in the years since

She and Brad divorced in 2005, though they’ve managed to keep up a cordial relationship in the years since.

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, Aniston and Pitt married. 

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

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

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 and Orlando showed off their need for speed 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’s jacket. 

Firm friends: Ellen DeGeneres was among the celebrity guests 

Firm friends: Ellen DeGeneres was among the celebrity guests 

Funny woman Ellen DeGeneres was also spotted joining the birthday shindig.

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.

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

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

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

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

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.

Chic in black: Kate Hudson stopped by the birthday party in a shining black dress

Chic in black: Kate Hudson stopped by the birthday party in a shining black dress

There was also plenty of Hollywood royalty at the Marly & 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.

Icons: There was also plenty of Hollywood royalty at the Marly & Me star's bash. Barbra Streisand and her husband James Brolin made an appearance

Icons: There was also plenty of Hollywood royalty at the Marly & 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

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.

Happy to be there: A smiling David Arquette, a figure from Jennifer's Friends days, made a special appearance

Happy to be there: A smiling David Arquette, a figure from Jennifer’s Friends days, made a special appearance

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