A deranged straphanger sucker-punched a subway conductor in Brooklyn on Tuesday — the latest in a recent string of assaults on MTA workers.

The unidentified man socked a Q train conductor after his Brighton Beach-bound train pulled into the Prospect Park station at 11:54 a.m., MTA officials said.

The rider was acting erratically while aboard the train, according to a witness, who said the lunatic got off at Prospect Park and, without warning, reached through a window and struck the middle-aged conductor in the head before fleeing the station.

“He just hit him and ran out,” said one stunned straphanger after the incident. “He was acting crazy in another car before it happened. I swear, man, it seems like there’s someone who just looks off on every train now. You never know.”

EMS was seen treating the conductor for a head wound. He eventually was taken to Kings County Hospital in an ambulance, FDNY officials said.

There have been at least seven assaults on MTA workers in the last 30 days, and transit union leaders say it’s because straphangers are frustrated over delays and poor service.

“I’m so shaken up,” said one visibly distressed MTA station worker who tended to the victim right after Monday’s incident. “We’re here doing our jobs and these things happen. It’s scary.”

There are no known arrests but the investigation is ongoing, police said.

Additional reporting by Stephanie Pagones and Max Jaeger