The 17-year-old singer celebrated Thursday on Instagram after sharing the new collaboration with fans.
Eilish, a longtime fan of Bieber, posted a throwback photo of herself wearing a rainbow sequined dress in her bedroom. Her bedroom walls were covered in posters of the "Sorry" singer.
"BAD GUY FEAT. JUSTIN BEIBER OUT NOWWW," she captioned the post. "OMGFFFFGGG ANYTHING IS POSSIBLE MAN."
Bieber shared the same picture of Eilish on his own account.
"@billieeilish so proud of you. Remix," he wrote.
The original "Bad Guy" appears on Eilish's debut studio album, When We All Fall Asleep, Where Do We Go?, which was released in March. The album also includes the singles "You Should See Me in a Crown" and "When the Party's Over."
Eilish urged fans to be open and proactive about their mental health in a PSA for Seize the Awkward, a mental health campaign, in May.
"It doesn't make you weak to ask for help," the star said. "You should be able to ask anyone for help, and everyone has to help someone if they need it."