The Riverhead Society spotted the shark on Dune Road Beach in Quogue, New York, on Thursday.
The eight-foot shark washed back into the sea before officers could secure it, Quogue Village Police Department said.
A statement reads: "At this time, we are cautioning swimmers and boaters in the area to be aware of this ongoing situation."
Later that evening, a 16-year-old surfer was bitten on his foot off Fire Island.
The teen was paddling around 20 yards off shore.
The eight-foot shark washed back into the sea before officers could secure it
Quogue Village Police Department
The Riverhead Society spotted the shark on Dune Road Beach in Quogue
Quogue Village Police Department
He was bitten on his right foot, and sustained a four-inch laceration.
Police say the 16-year-old was able to walk out of the water and was transported to the Good Samaritan Hospital Medical Centre for treatment.
A police helicopter was dispatched to the scene, but the water was too murky to locate the shark.
The incidents come after a string of sightings in the area.
Five people have suffered non-life-threatening injuries from shark attacks near Long Island beaches in the last month alone.
On Tuesday, beaches on the Rockaway Peninsula were shut down after a shark was spotted. No one was hurt.
Governor of New York Kathy Hochul has ordered enhanced shark monitoring and patrols.
