Police are looking for a man who punched a 5-year-old boy in the face while he was riding on a Brooklyn subway train with his mother.

The NYPD says it happened Saturday just before 4:30 p.m. The mother and son were ridding a southbound "G" train, approaching the Bergen Street station, when the stranger punched the little boy in the face for no apparent reason.

Police say the man then taunted the child before getting off the train and running away.

The boy sustained bruising and swelling on the left side of his face. EMS took him to NYU Langone Health Cobble Hill.

He was treated and released.

Police are looking for a man described as a black, 20 years-old, short black hair, mustache, 6' tall, 160 lbs., with a medium build. He was last seen wearing a olive colored long sleeve shirt, black jacket, black sneakers and beige cargo pants.

Police released photos taken by a witness.