While fans are thrilled about the new track, it seems like Eilish is over the moon to work with Bieber as she has been a long-time fan of the 'Sorry' singer.
In an Instagram post following the song's release, Eilish shared her excitement by sharing a throwback photo of her wearing a rainbow sequined dress while standing in front of her bedroom wall covered in posters of the 25-year-old pop singer.
"BAD GUY FEAT. JUSTIN BIEBER OUT NOWWW OMGFFFFGGG ANYTHING IS POSSIBLE MAN," she wrote alongside the picture.
Knowing how much the latest collaboration meant to Eilish, Bieber too shared the news on Instagram and Twitter by writing a sweet message for the 17-year-old star.
"So proud of you @billieeilish," he wrote beneath the same photo that Eilish had shared, before attaching their new song's link.
It's no secret that Eilish has been the biggest fan of Bieber for years.
During a November 2018 radio interview with KROQ's Kevin and Bean Show, as cited by People, the 'Ocean Eyes' singer referred to Bieber as her "first love."
"When I was, like 12, I was not a fan of him. That was, like, my first love. That was the person I was in love with. In my head, he was in love with me. It was, like, a relationship with a person," she said.
In March, the young star told Ellen DeGeneres she still has a soft spot for Bieber. "He's amazing. He's so sweet and, like, I feel -- just, honestly, I feel for him, man. He's been through a lot, dude," she said.
The same month, Eilish revealed that Bieber followed her on Instagram and even slid into her DMs with a throwback!
"The first thing he sent was a screenshot of a DM I sent him in 2014. You know how when you follow somebody, you can see everything they DM'd you in the past? It, like, it felt good and it felt bad at the same time," she told Entertainment Tonight, as cited by People.
But perhaps the biggest night for her was in April after Eilish performed at the 2019 Coachella music festival and eventually crossed paths with Bieber. During the annual music festival, Bieber commented on her Instagram with a "Yooo check ur dm gurl."
"So, this happened... Long and bright future ahead for you," Bieber captioned a photo of him and Eilish standing together during Ariana Grande's headlining set on April 14.
Bieber's manager, Scooter Braun, also captured the meeting and shared a video of it on Instagram the following day. In the clip, Bieber and Eilish shared a long hug, which Braun later called "magic" on his Instagram story.