Shirtless, tanned Justin Bieber and Mason Disick have fun pool day

Justin Bieber may have a very young new bestie! Kourtney Kardashian and Scott Disick's 5-year-old son Mason spent Saturday splashing around with the pop superstar in a pool. Justin initially shared the photo of Mason climbing on his back."Mason is cool as a fan," 21-year-old captioned the photo. The sweet picture was later shared by Kourtney and Mason's grandmother Kris Jenner.

Justin spent a sunny Saturday with 5-year-old Mason Photo: Instagram

"#Mason & @JustinBieber family pool day!!!" Kardashian matriarch Kris wrote alongside the photo. "#bestbabysitterintheworld."Justin is a close friend to Kris' youngest daughters Kendall and Kylie Jenner. The trio have traveled together and are often seen on each other's social media feed. Most recently, Justin joined Kendall in a photoshoot for Vogue's April issue. In the pics snapped by famed photographer Mario Testino, the duo had some fun of their own poolside. Also in the new Vogue dubbed "Brat Pack" are Gigi Hadid and Ansel Elgort. (click here to see how much fun BFFs Gigi and Kendall had with a selfie on a stick!)

Justin is no stranger to taking care of little kids. He takes his role as big brother to his two half-siblings Jaxon and Jazmyn seriously. The "Never Say Never" singer is often seen vacationing with them and spending as much time as he can with them.

That night, Justin surprised fans at Ariana Grande's Honeymoon tour stop in Miami. They sang a few songs including "As Long As You Love Me" and "All That Matters."