She captioned the shot: "BAD GUY FEAT".
Bieber teased the verse this week by tweeting the word "Remix."
"So proud of you", wrote Biebs on his Instagram, along with a photo of Billie as a tween in her natural environment - her bedroom, plastered with photos of JB.
BILLIE Eilish and Justin Bieber have worked together on a remix of her global hit Bad Guy.
Earlier this year, Justin was praising Billie on her performance at Coachella.
But as her career has taken off, one of the huge milestones she's just gotten to cross was a collab with her tween idol!
Eilish has made no secret about her feelings for Avril, recently telling Pitchfork, "Avril Lavigne was everything to me, dude".
The most recent pop star has been rocking the American levels in neons outfits and highlighted locks.
Billie Eilish is now on tour for most of the rest of the year 2019 and is playing a three-night stand this week in Los Angeles.
Justin's wife, model Hailey Bieber, also got in on the action, wrapping her arms around the Californian singer in a hug.