The 18-year-old singer was spotted courtside at the Staples Center in Los Angeles with Paul's namesake kid on his knee, reported Contactmusic.
"Justin appeared to be more than happy to hang out with young Chris. The pair were smiling, giggling and playing - and cheering on dad's team as they crushed the Celtics 106-77," a source said.
Bieber is no stranger to looking after little kids - as well as visits to members of his young army of fans, the 'Baby' singer has two siblings, Jaxon and Jazmyn, who are four and three respectively.