The 36-year-old victim was reportedly struck on the head following a dispute in Harlem on Tuesday morning – but miraculously survived without serious injury.
The man – whose vest was left soaked in blood – calmly received treatment for the head wound by emergency medics.
The victim had initially refused to go into the ambulance for treatment but was convinced by stunned onlookers.
“He looked like he didn’t even feel it,” one person told the New York Post.
The video was widely circulated on social media with the caption: “It’s going to be a crazy summer.”
The amateur videographer who recorded the scene is heard saying “he got stabbed” and that “it’s crazy out here”.
According to reports, the 36-year-old man was struck in the head after he intervened in an argument between a woman and another man.
He was listed as being in a stable condition at Harlem Hospital.
One security guard said: “I’m traumatised from seeing that. It looks like a scene out of a freaking movie.”
No arrests or charges have yet been made, according to the New York Police Department.
Critics have accused politicians of losing control of law and order in New York in recent months following a spate of shootings and killings in recent months.
Ed Mullins of the Sergeants Benevolent Association reacted to the video in a tweet on Tuesday: “NYC politicians have lost control of the city. One day a homeless man is set on fire in Manhattan another day a man is knifed in the head, all on the heels of 125 shootings in one month.”
New York City Mayor Bill de Blasio has pledged to crack down on crime in the city which has seen a dramatic fall in crime after its high-point in the 1980s and 1990s.
He said: “We’re not going back to the bad old days when there was so much violence in this city”
“We are not going to allow gun violence to continue to grow in this city.”