A good Samaritan was attacked on the subway when he confronted a stranger who was bullying another straphanger, cops said.

The man was punched in the head on a Brooklyn-bound A train in Lower Manhattan around 10:45 p.m. Thursday, police said.

The victim had spotted a man harassing another rider on board the train and intervened — sparking a dispute, according to cops.

The argument turned violent when the brute socked the do-gooder in the head, authorities said.

The attacker bolted from the train at Spring Street, cops said. 

The victim got off the train at Fulton Street and walked into NewYork-Presbyterian Lower Manhattan Hospital, where he was treated for a cut on his head, cops said. 

No arrests had been made Friday.