The 48-year-old victim was arguing with another straphanger on the train about 1:38 a.m. when the suspect intervened, only to get embroiled in the spat himself, police said.
When the train pulled into the Clinton-Washington Avenues station, the suspect got off the train and walked to the door where the victim was standing, slicing his arm just before the doors closed, an NYPD spokesman said.
The victim managed to get help when he got off the train at the Lafayette Avenue station, police said. He was treated for serious injuries at Methodist Hospital, where he's in stable condition, police said.
The suspect wasn't immediately arrested, police said.