The incident occurred on Thursday morning in the East New York neighborhood in Brooklyn, ABC New York station WABC reported.
Sgt. Ritchard Blake shot the man because they had been disputing over a woman, police sources told ABC News.
The video begins as Blake walks forward but turns around to talk to the man, who then walks into view.
The pair keep their distance at first as they talk, but the man can be seen slowly inching toward Blake, who gesticulates slightly with his hands during the conversation.
As the man continues to walk toward Blake, the police sergeant then appears to pull a gun out of his waistband and fires multiple rounds at the man, shooting him point-blank.
Blake then approaches the man, who is lying on the ground, while still pointing the gun at him. He then conceals the firearm in his clothing and pulls out his phone before patting the man's left pocket.
The police sergeant could also be seen placing an object near the man and then picking it up seconds later.
The NYPD is reviewing the video and coordinating with the Kings County District Attorney's Office on an investigation, the department said in a statement.
"The video captures actions that raise serious questions, and require further investigation," the statement read.
The man -- identified by WABC as 21-year-old Thayvone Santana -- is in stable condition at the hospital, police sources said.
Blake told his superiors at the police department that Santana tried to rob him, police sources said. Blake was on his way to work at the time and told officers responding to the scene that the man was a stranger, WABC reported.
Blake also allegedly attempted to plant a sharp object on Santana took it back after looking up and realizing that there was a surveillance camera nearby, police sources said.
Santana's family told WABC that the two had been fighting over a woman for weeks.
Blake has been placed on modified duty, the NYPD said in a statement. He has been on probation after he was arrested two years ago due to an incident with his girlfriend at the time, according to WABC.
"There are certain things that we saw in this investigation that we have questions that we want answered at this point, until we answer that we felt it was best to place him on modified duty," said NYPD Chief of Department Terence Monahan, according to WABC.
New York City Sergeants Benevolent Association President Ed Mullins said in a statement to WABC that Blake is "entitled to due process."
"It is crucial for the integrity of the law enforcement community, as well as the Sergeants Benevolent Association, that all of the facts are obtained and thoroughly investigated," Mullins said. "As the nuances of this unique case are reported, it is also important that the good work that police officers of all ranks do every day not be diminished in any way."