Once inside, the seven-time Grammy winner packed on the PDA with her 31-year-old beau, who couldn’t keep his hands off her as they kissed on the dance floor before her loved ones gathered to sing her Happy Birthday.

Smitten: Billie Eilish got into the festive spirit as she slipped into a 𝑠e𝑥y Mrs. Claus costume to celebrate her 21st birthday , attended by her boyfriend Jesse Rutherford, former celebrity crush Justin Bieber and his supermodel wife Hailey

As a DJ asked her friends and family to put their ‘mother f**king hands in the sky for Billie’ and ‘make some noise,’ the lead singer of the rock band The Neighbourhood, kept his arms wrapped his arms around her shoulders.

As the crowd sang and presented her with a beautiful birthday cake, the Gen Z superstar beamed next to her man, gazed lovingly at her as she prepared to blow out her candles, and rapper Doja Cat.

Rutherford rocked a seasonal Santa suit, which coordinated with Eilish’s skimpy Mrs. Claus outfit, consisting of a  faux fur-trimmed minidress, matching elbow-length gloves, a black belt and large silver hoop earrings.

As the Oscar winner’s guests serenaded her, she joyously held up one of her first legal drinks, alongside some of Hollywood’s biggest celebrities.

Birthday girl: While arriving to her bash in a bright red sled, alongside a friend holding a champagne flute, the Bad Guy singer looked blissful as she arrived to at The Edition in West Hollywood with an unidentified buddy on Saturday night

Dancing queen: Once inside, the seven-time Grammy winner packed on the PDA with her 31-year-old beau, who couldn’t keep his hands off her as they kissed on the dance floor before her loved ones gathered to sing her Happy Birthday

Cozy: As a DJ asked her friends and family to put their ‘mother f**king hands in the sky for Billie’ and ‘make some noise,’ the lead singer of the rock band The Neighbourhood, kept his arms wrapped his arms around her shoulders

The attendees also included Kendall Jenner, Olivia Rodrigo, pop-punk princess Avril Lavigne, Dove Cameron, Eiza Gonzalez, drummer Gethin Davies and more.

Jenner, 27, and Hailey Bieber, 26, appeared to have a blast at the event while playfully posing with middle fingers in a photo booth.

Kendall also posted some sultry sepia shots of herself on Sunday, capping off a busy weekend for the supermodel as she celebrated her nephew Mason’s epic 13th birthday earlier in the evening.

Yum! As the crowd sang and presented her with a beautiful birthday cake, the Gen Z superstar beamed next to her man, gazed lovingly at her as she prepared to blow out her candles, and rapper Doja Cat

Coordinating: Rutherford rocked a seasonal Santa suit, which coordinated with Eilish’s skimpy Mrs. Claus outfit, consisting of a faux fur-trimmed minidress, matching elbow-length gloves, a black belt and large silver hoop earrings

Twenty-fun! As the Oscar winner’s guests serenaded her, she joyously held up one of her first legal drinks, alongside some of Hollywood’s biggest celebrities

The models also snapped photos with Jesse Jo Stark, 31, which showed them flashing cheesy grins, sticking out their tongues and smooching their lips.

Eilish’s party came on the heels of her Friday night performance at the Kia Forum in Ingelwood, California, where the singing sensation was presented with a cake on stage by her tight-knit family and new boyfriend Jesse Rutherford, 31.

‘Want to take shots?’ Billie joked from the stage. ‘Give me two days. I am 21 in two days, it’s f••king unbelievable.’

Life of the party: The attendees also included Kendall Jenner, Olivia Rodrigo, pop-punk princess Avril Lavigne, Dove Cameron, Eiza Gonzalez, drummer Gethin Davies and more

Cheers: Everyone at the party raised their glasses to the birthday girl

As she let loose at her party, Eilish did not shy away from engaging in some PDA with her boyfriend, after years of keeping past romances with Matthew Vorce, Brandon Que and Henry Whitford out of the spotlight.

Earlier this month, she explained that ‘falling in love is hard’ for her during an interview with Highsnobiety.

‘It’s a spooky thing, love. It’s really spooky,’ the performer confessed.

Festive: The bash was holiday-themed

Eilish also addressed the scrutiny her relationship has come under due to their 11-year age gap.

‘Say what you need to say, but, like, I am in control right now. I did this,’ she told the publication.

‘I managed to get my way 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**! Are you kidding me? Can we just [get a] round of applause for me?’ she marveled.

Arriving in style: Eilish arrived in a sled with a drink in hand

Just getting started: The night seemed to get better and better for Eilish and her guests

She continued: ‘I pulled his a**! All me! I did that s**t! I locked that motherf**ker down!’

Eilish also addressed the scrutiny her relationship has come under.

‘Say what you need to say, but, like, I am in control right now. I did this,’ she told the publication.

Happy Birthday Billie! Jenner, 27, and Hailey Bieber, 26, appeared to have a blast at the event while playfully posing with middle fingers in a photo booth

Not just a third wheel: Kendall and Bieber’s close friend Jesse Jo Stark, 31, joined them in the photo booth

Photo booth fun: The three friends ran through the typical array of classic photo booth looks, flashing cheesy grins, sticking out their tongues, smooching their lips and flipping off the camera

Sweet! Hailey shared a snap of her kissing husband Justin

More from the photo booth: Eiza Gonzalez and Hailee Steinfeld posed for pictures together

Eilish also revealed her love language was physical touch as she detailed the attributes she needed in a relationship.

‘I just need to be touching skin, all the time. Touching, and cuddling, and hugging, anything skin related is really a big thing for me. And other than that, just freedom. You know, I don’t want to be controlled. I want to be trusted and I want to be able to have space and I want love and attention and equal admiration is really important.’

The musician also said she was ‘extremely grateful’ for her past relationship with Vorce.

New romance: Billie and Jesse made their red carpet debut at the star-studded 2022 LACMA Art+Film Gala last month

‘I feel extremely grateful for the relationship that I was in last year and I think he’s f**king so much happier now, which makes me really happy,’ she said.

Billie and Jesse made their red carpet debut at the star-studded 2022 LACMA Art+Film Gala earlier this month.

Though the pair were in high spirts, they were recently slammed over their ‘disturbing’ baby and old man Halloween costumes online.

Sparking controversy: Though the pair were in high spirts, they were recently slammed over their ‘disturbing’ baby and old man Halloween costumes online