Just a day after releasing her "Bad Guy" remix with another musical hero, Justin Bieber, the singer came face to face with the "Complicated" hitmaker.
Posting a photo she took with the smiling Canadian on Instagram, Billie wrote: "THANK YOU FOR MAKING ME WHAT I AM."
Eilish has made no secret about her feelings for Avril, recently telling Pitchfork, "Avril Lavigne was everything to me, dude."
"I love Avril Lavigne. I only love her. There's nothing else there. Just love. Only love. There's, like, almost no artists like that. Like Avril Lavigne and Justin Bieber, I only love."
Avril wasn't the only celebrity at the show in Los Angeles - Billie's superfan Melissa McCarthy was also spotted in the audience.