Barnegat Bay is a small brackish arm of the Atlantic Ocean, approximately 30 miles long, along the coast of Ocean County, New Jersey in the United States.

A bay is defined as an indentation of a shoreline larger than a cove but smaller than a gulf. Barnegat Bay would more correctly defined as a Lagoonal Estuary. 

A Lagoon is defined as  a body of  shallow salt or brackish water separated from the deeper sea by a shallow or exposed barrier beach, sandbank of marine origin or similar feature.

An Estuary is defined as a zone along a coastline where freshwater systems and rivers meet and mix with a salt water.

Barnegat Bay is separated from the Atlantic Ocean mainly by a  peninsula, Island Beach and an island, Long Beach Island.  The ocean waters enter the bay though three inlets, Barnegat Inlet in the center, Point Pleasant Canal to the north and Little Egg Inlet to the south.  <see map>

Fresh water enters the bay primarily from rivers (Metedeconk River, Toms River, Forked River the Mullica River) and streams. Also from ground water and surface water runoff.

The bay is part of the Atlantic Intracoastal Waterway, entered on its north end by the Point Pleasant Canal and connecting on the south end with Little Egg Harbor via the small Manahawkin Bay.