The Riverhead Foundation for Marine Research and Preservation says it received several calls about a distressed seal pup on Saturday morning.
The female pup was struggling against the surf at The Sands, a banquet facility in Atlantic Beach. The pup was 0.6m long and her umbilical cord was still attached.
She was taken to the foundation's marine mammal hospital in Riverhead, where she's being fed formula.
It's uncertain if the pup was abandoned or if crowds of people on the beach may have kept the mother away.
Harbour seals are protected under the federal law, and it is unlawful to approach, handle or feed them.