The Grammy-winning artist, whose birthday falls on December 18, enjoyed a wild night out with boyfriend Jesse Rutherford, 31, as well as Justin Bieber, 28, Bieber’s wife Hailey, 26, and rapper Doja Cat, 27.
The star, who is finally of legal drinking age in America, arrived at the occasion at The Edition in Hollywood on a Christmas Santa sleigh and danced the night away once she was inside.
Eilish and Rutherford wore matching Santa Claus and Mrs Claus outfits as they drank into the night and were accompanied by many more famous faces.
Kendall Jenner, Dove Cameron, and pop-rock singers Olivia Rodrigo and Avril Lavigne were also invited to the Los Angeles birthday bash.
In one video posted on Instagram by Lavigne, 38, the DJ at the party can be heard asking the guests to ‘put [their] hands in the air for Billie’.
Later in the night, another video – re-posted to Eilish’s Instagram story – shows the party-goers singing a loud rendition of Happy Birthday to the Bad Guy singer.
Before the weekend celebrations, Eilish performed to a sell-out crowd at the Kia Forum in Inglewood, LA.
On stage, she was presented with a cake by Rutherford and the rest of her family, before joking about immediately buying shots once she turned 21.
‘Want to take shots? Give me two days. I am 21 in two days, it’s f***ing unbelievable,’ she laughed with her audience.
Eilish’s lifelong friend Annabel Zimmer posted a photobooth snap of the two, captioning the post: ‘Today, you were born and that is very cool and nice and good.’
Other famous party attendees also made good use of the party’s photobooth, with Kendall Jenner and Hailey Bieber posing for pictures as well.
Eilish’s brother Finneas, who has spent the last few months recovering from a biking accident, was also in attendance – he’s thankfully on the mend.
In a post on Twitter, PopCrave detailed the full list of stars who attended the lavish event and featured the video of them all loudly singing Happy Birthday to Eilish.
Last month, Eilish defended her relationship with singer-songwriter Rutherford, saying she is ‘inspired’ by him and ‘excited’ by their bond.
‘I just am really inspired by this person, and, you know, he’s inspired by me. It’s really cool, and I’m really excited and I’m really happy about it,’ she said.
She then went on to state that she was the one who approached Rutherford, frontman of the band Neighborhood, as opposed to the other way around.
Eilish said: ‘I managed to get to a point in my life where I not only was known by a person that I thought was the hottest f**king f**ker alive, but pulled his a**!’
She admitted that she couldn’t believe her luck when the two began dating: ‘Are you kidding me? Can we just [get a] round of applause for me?’
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