The shooting happened just before noon at the C-Town Supermarket store on Southern Boulevard in the Mott Haven section.
According to the NYPD, the victim and another man were inside the store when another person walked inside and started firing shots at them.
Video from inside the C-Town released by the NYPD:
The 21-year-old victim was hit three times, with bullets striking his back and leg, police said.
The suspect fled on foot southbound on Southern Boulevard.
The victim was taken to the hospital in stable condition.
No other details have released.