The incident began inside a McDonald's on Mermaid Avenue in Coney Island and spilled out on the street.
The two victims were stabbed just outside the restaurant around 11 p.m.
A 20-year-old man was pronounced dead on arrival at Coney Island Hospital.
The other victim, 20, suffered a stab wound in the back and was in stable condition at Lutheran Medical Center.
No arrests were immediately made.
The investigation is ongoing.
The identity of the deceased is pending family notification.