A straphanger was stabbed in the face with a screwdriver when he stepped in to break up a fight on a Brooklyn subway train, police said.

The 37-year-old victim was riding a Manhattan-bound R train approaching the 25th Street station late Sunday, just before midnight, when he saw a man and woman brawling, cops said.

The good Samaritan tried to mediate the clash — which is when the man involved in the tussle pulled out the screwdriver and stabbed him on the left side of the face, authorities said.

The victim suffered a cut on his left cheek and earlobe, according to cops, who said he got off the train at the 25th Street station and called 911.

He was taken to a hospital for treatment and expected to recover.

The attacker and the woman he’d been brawling with appeared to know each other — but were both strangers to the victim, cops said.

The suspect and his female companion were still on the loose Monday.