A man was fatally stabbed on an NYC subway Wednesday … and insane video shows the aftermath as bystanders chase after the knife-wielding suspect.

According to police, 25-year-old Jyrell Thomas was acting agitated and pacing inside the J train before walking up to fellow passenger Mesias Lojano and swinging at him with a kitchen knife.

A bystander on the train began filming on his phone amid the chaos … recording as he chased after the suspect through the platform and down the street.

The clip is crazy … the bystander yells out for someone to grab the weapon as the pursuit eventually leads to a parking lot, where the suspect hops the fence and winds up on a nearby roof.

CBS News reports that police sources initially said the attack was unprovoked, but later said Lojano had gotten into an argument before he was stabbed multiple times in the neck and chest.

The outlet reports he was found unconscious and unresponsive at the subway station. He was taken to a hospital where he was pronounced dead.