The so-called “ninja” who allegedly attacked a straphanger with a sword sheath on a Manhattan subway train Thursday has been arrested, police said.

Selwyn Bernardez was arrested Friday afternoon on the corner of Lispenard Street and West Broadway in Lower Manhattan, less than half a mile from where he allegedly smacked his 29-year-old victim over the head with the encased weapon, the NYPD said.

The 27-year-old Brooklyn man was charged with assault and reckless endangerment for his alleged crime.

The man Bernardez allegedly attacked happened to be Larry Griffin II, who is on parole for leaving at least two rice cooker bombs in the Fulton Street subway station in 2019.

Griffin followed Bernardez into a northbound A train as it approached the Chambers Street Station and got into a dispute with him at 9:30 a.m., police sources have told The Post.

Both men got off and Bernardez allegedly hit Griffin over the head, leaving him with a deep gash, police and the sources said.

Bernardez, who was wearing an all-black outfit, hat and sunglasses, bolted from the scene, leaving his weapon behind.

Mayor Eric Adams said the ordeal was an example of the city’s enduring mental health crisis.

“When you do an analysis of the subway crimes we are seeing, you are seeing it is driven by people with mental health issues,” Adams said Thursday.

“If you got a ninja outfit on and you are running around with a sword, then something is wrong.”

Police have not disclosed what provoked the dispute between Bernardez and Griffin.