A ski mask-wearing maniac stabbed and bit a straphanger during a violent Friday afternoon scuffle on board a Brooklyn train, cops said.

The 58-year-old victim was riding a Manhattan-bound L train near the Graham Avenue station in Williamsburg when he got into a heated fight with another rider, cops said. 

The unhinged stranger then stabbed the man in the back of his shoulder and bit him on the top of his head, police said. 

The victim was taken to Bellevue Hospital, where he was listed in stable condition, authorities said. 

The alleged attacker, last seen wearing all black with a black ski mask, fled on foot. 

He had not been caught by Friday evening.