Police are investigating the death of a man who was allegedly harassing subway riders in NoHo before one passenger took matters into his own hands.

Officials say the altercation happened at around 2:30 p.m. Monday on board the F train at the Broadway-Lafayette Street station.

A 24-year-old passenger stepped in to subdue an older man who was acting erratically toward fellow riders. The harasser then fell unconscious, officials said.

Authorities say he was taken to Lenox Hill Hospital where he was later pronounced dead.

Police say the man who died was a subway recidivist with 44 prior arrests for assault, disorderly conduct, and fare evasion.

The man who attempted to subdue the alleged harasser was questioned by detectives and released pending results of an investigation and autopsy, officials said.