A 13-year-old boy took a bullet to the chest when someone opened fire on a Bronx school yard Monday night, police said.

The youngster was behind the School of Science and Applied Learning on Prospect Avenue near East 179th Street at about 10:20 p.m. when the gunman started spraying bullets, cops said.

He was rushed in a private car to St. Barnabas Hospital, where he’s in serious but stable condition, authorities said.

The gunman ran off after the shooting and has not been arrested, cops said.

Police are investigating the circumstances, but do not believe the teen was an innocent bystander, a law-enforcement source said.