"The Keeping Up With The Kardashians" star Kris Jenner says "Baby" singer Justin Bieber is the "best babysitter in the world".

That appears to be Kris’ way of thinking as she headed to image-sharing website Instagram on Saturday afternoon to show off a picture of Bieber and Kourtney Kardashian’s five-year-old son Mason Disick having fun in the pool, reports eonline.com.

"#Mason & @JustinBieber family pool day!!! #Bestbabysitterintheworld (sic)," Kris captioned the image.

Bieber himself didn’t seem to hate his afternoon with Kourtney’s oldest son either. Wearing a baseball cap and his signature shades, the crooner wrote: "Mason is cool as a fan."

The Kardashian family and Bieber have a long history and friendship together. For a time, Kourtney was neighbours with the pop star.

There’s also a special bond that Kendall Jenner and Kylie Jenner share with the singer. Whether posing in photoshoots together or hanging out together at mutual friends’ parties, the threesome have remained friendly for many years.