A man was stabbed on a subway train in the Bronx on Sunday night after he stepped in to defend a woman who was being harassed, cops said.

The 54-year-old rider witnessed the woman being bothered on a southbound 4 train at around 9 p.m. He tried to intervene and became the target of the man’s fury, according to police.

The 42-year-old suspect then pulled out a knife and stabbed the Good Samaritan as the train pulled into East 149th Street & Grand Concourse station.

The man was stabbed in the left elbow and taken to Lincoln Medical Center for treatment.

Police arrested the suspect at the scene. Charges against him were pending early Monday.