Oh baby, baby! At iHeartRadio's 102.7 KIIS FM 2019 Jingle Ball on Friday, Billie Eilish paid tribute to none other than Britney Spears.

The 17-year-old singer wore a T-shirt and basketball shorts bearing the teen pop princess-turned-queen's likeness and name while performing at the event, which took place at the Forum in Inglewood, California. Gen-Zer Billie may be unfamiliar with Van Halen and Hilary Duff, but she knows and loves her Britney.

Perhaps Billie was simply returning the favor; In June, Britney posted on Instagram a video of herself dancing to Billie's "Bad Guy" while recreating her iconic "I'm a Slave 4 U" choreography from the 2001 MTV VMAs. The teen, who was born three months after that ceremony, commented, "OMG."

The Jingle Ball took place less than two weeks before Billie turns 18.

"I think it's really hard to navigate your teens and let alone navigate them as a person with a lot on their plate, with a lot to do. I think she's done it incredibly well and I'm just really proud of her," her brother, fellow musician, songwriter and producer Finneas O'Connell told E! News exclusively at the show. "I couldn't imagine a person turning 18 with more to be proud of."

Billie was also jointed at the event by her parents.

Other performers at KIIS-FM's Jingle Ball included Camila CabelloLizzoKaty PerryBTS with special guest HalseySam Smith with special guest Normani and Louis Tomlinson, who replaced French Montana, who was recently hospitalized over concerns for his cardiovascular health, and is now on bed rest.

Lizzo stepped on stage wearing a pair of Daisy Dukes and a Santa-like coat.

"Close your eyes and say, 'I love you, me,'" she told the crowd. "Now look at me and say, 'I love you, Lizzo. Now this is a very no judgment crowd, no shame in your game. If you want to wiggle, I want to see you wiggle. If you want to laugh, then laugh. If you want to scream, scream!"

Later in her set, she told the crowd, "Thank you for accepting me for who I am. I want to let you know that if 20,000 people can accept me for who I am, the whole world can fall in love with you."

Celebrity attendees included Jamie Foxx, former Real Housewives of Orange County star and newly single Meghan King Edmonds—who attended with a girlfriend, Real Housewives of Beverly Hills stars Kyle Richards and Teddi MellencampThe Bachelorette alum Hannah BrownThe Bachelor alum Chris Soules, Bachelor Nation couples Caelynn Miller-Keyes and Dean Unglert and Ashley Iaconetti and Jared Haibon, plus Jason Priestley and Sarah Michelle Gellar.

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Camila Cabello, Jingle Ball

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Lizzo

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Taylor Swift, 30th Birthday, Jingle Ball

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iHeartRadio Jingle Ball 2019, Lizzo, Billie Eilish, Katy Perry, Sam Smith, Camila Cabello, Normani

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iHeartRadio Jingle Ball 2019, Lizzo, Billie Eilish, Katy Perry, Sam Smith, Camila Cabello, Normani

Joseph Llanes for iHeartRadio

iHeartRadio Jingle Ball 2019, Lizzo, Billie Eilish, Katy Perry, Sam Smith, Camila Cabello, Normani

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Katy Perry, Jingle Ball

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iHeartRadio Jingle Ball 2019, Lizzo, Billie Eilish, Katy Perry, Sam Smith, Camila Cabello, Normani

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iHeartRadio Jingle Ball 2019, Lizzo, Billie Eilish, Katy Perry, Sam Smith, Camila Cabello, Normani

Joseph Llanes for iHeartRadio

iHeartRadio Jingle Ball 2019, Lizzo, Billie Eilish, Katy Perry, Sam Smith, Camila Cabello, Normani

Joseph Llanes for iHeartRadio

iHeartRadio Jingle Ball 2019, Lizzo, Billie Eilish, Katy Perry, Sam Smith, Camila Cabello, Normani

Joseph Llanes for iHeartRadio

iHeartRadio Jingle Ball 2019, Lizzo, Billie Eilish, Katy Perry, Sam Smith, Camila Cabello, Normani

Joseph Llanes for iHeartRadio

iHeartRadio Jingle Ball 2019, Lizzo, Billie Eilish, Katy Perry, Sam Smith, Camila Cabello, Normani

Joseph Llanes for iHeartRadio

iHeartRadio Jingle Ball 2019, Lizzo, Billie Eilish, Katy Perry, Sam Smith, Camila Cabello, Normani

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2019  iHeartRadio Jingle Ball, Billie Eilish

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2019 iHeartRadio Jingle Ball, Billie Eilish

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2019 iHeartRadio Jingle Ball, Camila Cabello

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2019 iHeartRadio Jingle Ball, Halsey

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2019 iHeartRadio Jingle Ball, Louis Tomlinson

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2019 iHeartRadio Jingle Ball, Normani

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2019 iHeartRadio Jingle Ball, Camila Cabello

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2019 iHeartRadio Jingle Ball, Hannah Brown

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2019 iHeartRadio Jingle Ball, Kyle Richards

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The singers showed each other much support; Lizzo bopped her head to BTS' music. Halsey danced, sang along and air-drummed to Katy's performance of "Firework." Louis danced backstage to Camila's performance of "Havana."

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