Justin Bieber knows that exes and parties don't always mix.
The Purpose singer showed up to Rihanna's GRAMMY after-party at the 1Oak night club in West Hollywood, California, on Sunday night -- where his ex Selena Gomez was spotted out partying with her new guy, The Weeknd.
MORE: Selena Gomez Parties With The Weeknd After GRAMMYs, While Justin Bieber Continues to Diss 'Starboy' Singer
Bieber left shortly after arriving, which could have to do with seeing his ex at the club; an eyewitness tells ET Selena was getting super close and intimate the whole night with The Weeknd.,
Earlier on Sunday, Bieber took to Instagram Live to continue shading The Weeknd, sarcastically referring to "Starboy" as his favorite song, before he and his friends burst out laughing.
We gotta end it now, y'all. Y'all, that was too funny," the 22-year-old pop star continued.
WATCH: Must-See Moments From GRAMMYs After Parties
Meanwhile, Bieber's been doing his own thing after returning to Instagram!
Watch the video below to see the singer's recent shirtless pics.