A straphanger was stabbed on a Brooklyn J train while trying to help a fellow passenger early Sunday — the second knifing on the subway line in a matter of days, law-enforcement sources told The Post.

The suspect was trying to flirt with a woman on board the train just after 2 a.m. at the Myrtle Avenue/Broadway stop in Bed-Stuy when another male rider intervened, sources said.

The two men began arguing, and the rowdy would-be Romeo knifed his 31-year-old victim in the chest and torso, according to sources.

The victim was taken to Kings County Hospital in stable condition while the suspect is large, according to the NYPD.

The scary incident comes just days after a fatal stabbing on the J train in Williamsburg, Brooklyn.

That victim, ex-con Devictor Ouedraogo, was harassing commuters on the train as it approached Marcy Avenue, including alleged attacker Jordan Williams’ girlfriend, prosecutors said.

Ouedraogo of Bedford-Stuyvesant had asked the woman, “Want to f–k?” and kept harassing her even though Williams told him to get lost, according to prosecutors.

A scuffle ensued, with Ouedraogo slugging Williams’ girlfriend in the face — before the boyfriend fatally stabbed him in the chest.

Williams was charged with manslaughter and criminal possession of a weapon and released without bail. He faces up to 26 years behind bars if convicted.

“I really do not believe that you pose a flight risk – I just don’t,” Brooklyn Criminal Court Judge Sherveal Mimes told Williams at his arraignment Thursday. “I think that your whole life is ahead of you. I think that you have every reason to fight this case with the support of your family.”

The violence underground also follows the death of Jordan Neely on an F train in May. The homeless man who went on a threatening rant before he was placed in a fatal chokehold by former Marine Daniel Penny.