Posting to his Instagram story, the Biebs criticized Em for insulting younger hip-hop artists on 2018's Kamikaze.
"I just like Em's flow but don't like that he's dissing new rappers," he wrote following a screenshot of the album's opener, "The Ringer."
"I like the new generation of rap he just doesn't understand it," he added.
As Complex notes, the track is largely a critique of today's so-called "mumble rappers."
Eminem sniped at newbie hip-hop stars on the track (with a few notable exceptions), rapping, “I heard your mumblin’ but it’s jumbled in mumbo-jumbo / The era that I’m from will pummel you, that’s what it’s comin’ to / What the f*ck you’re gonna do when you run into it? / I’m gonna crumble you and I’ll take a number two / And dump on you, if you ain’t Joyner / If you ain’t Kendrick or Cole or Sean, then you’re a goner.”
Eminem also dissed Tyler, the Creator on "Fall" and MGK on "Not Alike."
Bieber himself got a benign shout-out on Eminem's 11-minute freestyle "Kick Off," which was released late last year.
"Justin Bieber in a Catholic school/Selling dust and reefer when your back is to him," Em rapped.
Unfortunately for the Canadian pop star, Twitter users are now roasting him for criticizing Eminem's understanding of the very genre he's been a dominant force in for two decades.
That said, a couple of people did agree with the Biebs' comments.
Are you #TeamBieber or #TeamEminem?