Billie Eilish, a Belieber, recaps her starstruck Justin Bieber encounter

Billie Eilish is a Belieber.

On Thursday’s installment of “Carpool Karaoke,” the music phenom recapped an ultimate fan moment: Meeting pop star Justin Bieber at last April’s Coachella Valley Music and Arts Festival — and how she had no chill.

“For all Coachella weekend, I was like, ‘Don’t surprise me with Justin Bieber. I can’t take it. I can’t have to do this show and Justin Bieber’s going to be there,’” she told “Late Late Show” host James Corden as she accompanied him for the carpool bit.

The “Bad Guy” singer said the pop summit occurred unexpectedly while she was watching Ariana Grande perform. It left her completely starstruck.

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“I saw [him]. I know what Bieber looks [like]. I know his body language. I know how he stands. I know where he wears his pants,” she explained. “So I look over and he just stood there. Like five feet away from me. Perfectly still. And he had the face mask so all I could see were his eyes.”

“And you lost your brain,” Corden interjected.

“And he lost my brain,” she said, clearly speechless.

From there, Eilish and the music-loving Corden segued into the Biebs’ career-making hit “Baby,” with Eilish rapping the lyrics from Ludacris’ verse.

Also while sitting in Corden’s passenger seat, the 18-year-old sang a slew of her own tunes, including “Bad Guy,” “All the Good Girls Go to Hell,” “Ocean Eyes” and “When the Party’s Over.” She was emotional listening to them play back as she took in the life-changing gravity of each song. She also performed lovely ukelele versions of the Beatles’ “I Will” and a few soulful ditties she wrote with a friend when she was 7 years old.

The L.A.-bred artist took Corden on a tour of her Highland Park home, where he met her mom and a creepy pet tarantula, and spent time in the room where she and brother Finneas O’Connell created the Grammy-nominated album “When We All Fall Asleep, Where Do We Go?” (She still lives there with her parents, by the way.)

The green-haired singer and her brother are each up for Grammy Awards next month. Eilish, who was nominated for six awards, made Grammy history last month by becoming the youngest person to be recognized in the top four categories: new artist, album, record and song of the year.