The 21-year-old singer -- who was partying in Miami, Florida, earlier this week -- gave yet another impromptu performance at the swanky celebrity hot spot. An eyewitness tells ET that he arrived around 9 p.m. and hopped on the piano to play a few of his newest hits, including "What Do You Mean?", as guests were eating dinner.
An eyewitness told ET that Justin was "very friendly," saying hello to people in passing and as fans approached him at the piano.
"He was so relaxed," the eyewitness said. "I've never seen him this calm. It was nice to hear him play the piano."
"I had no idea he was going to be here, so it was a nice surprise!" the source added.
Another celebrity in attendance was Kourtney Kardashian. Rumors have continued to swirl around the 36-year-old Keeping Up With the Kardashians star and her longtime pal, Bieber. A source told ET that they "were nearly inseparable" throughout the night. The venue seems to be a favorite for these two, as they were also spotted there together in October.
Bieber later took to Instagram to share snippets from his intimate musical session.
Fans in attendance at the unexpected show also shared snaps from the evening. Some even posed by the Canadian pop star's brand new, bright blue Ferrari, which he apparently parked illegally and was ticketed for.
The new whip made a debut on Bieber's Instagram a few hours after the performance. A very humble caption read, "Honestly im not trying to show off!! I am so proud and grateful of the things that I have been blessed with and don't take it for granted."
"I feel so blessed that I STILL get excited and ask my friends to take a picture of me with it," he added. "Never think I'm trying to throw it in your face!!"
While Bieber was in Los Angeles, his ex, Selena Gomez -- who he's been causing a lot of buzz about thanks to a series of flashback posts and comments -- was in New York City. The former Disney star reunited with one of her other ex-beaus, Zedd, as the two took the stage at Z100's iHeartRadio Jingle Ball to perform their song "I Want You to Know" together.
Gomez took to Instagram to share a photo from the stage, writing, "My babes @zedd and I performing tonight for @z100newyork @iheartradio. SO grateful to wrap up this year with the best people. Thank you NY!!"
"Reunion with my favorite lil Mexican taco," Zedd captioned a snap of the two.