Jamaica Bay is an estuary on the southern portion of the western tip of Long Island, in the U.S. state of New York. The estuary is partially man-made, and partially natural. The bay connects with Lower New York Bay to the west, through Rockaway Inlet, and is the westernmost of the coastal lagoons on the south shore of Long Island. Politically, it is primarily divided between the boroughs of Brooklyn and Queens in New York City, with a small part touching Nassau County.

Lower New York Bay is a section of New York Bay south of the Narrows. The eastern end of the Bay is marked by two spits of land, Sandy Hook, New Jersey, and Rockaway, Queens. The waterway between the spits connects the Bay to the Atlantic Ocean at the New York Bight. Traversing the floor of the Bay southeasterly from the Narrows to the Bight and beyond is Hudson Canyon.