A woman repeatedly slapped a stranger aboard a Brooklyn subway car as he cried in vain for help, a disturbing video shows.

Cellphone footage shows the incident play out on an R train Saturday, according to Newsflare.com, which published the 33-second clip.

In the video, a woman is seen approaching a man seated on the train and hitting him in the head, saying: “Why you act mother­f–ker?”

“I didn’t act anything!” the startled man shrieks.

Looking around the train car for help, the man pleads, “Call the police! Please, call the police!”

The woman then whacks the man in his body before slapping the side of his head, the video shows.

“Stop … I’m calling the police. I’m taking video of you,” the man warns before the woman slaps him again.

The attacker tells the man, “Call the police” as she again hits him.

“I’m calling them! Help me! Someone call the police! Someone call the police,” the man calls out to no avail.

The woman gets another slap in, striking the man in the face before the clip ends.

“Stop! Stop!” the victim shouts.

The eyewitness who shot the clip told Newsflare the incident began when the man said something that apparently incensed the woman.

“I was on the train and all of a sudden this woman started yelling at the man cause he said something wrong and she got up and started attacking him,” the person told the video agency.

The MTA and the NYPD did not have any information on the incident.

Additional reporting by Danielle Furfaro and ­Stephanie Pagones