Lagoon, a relatively shallow body of water separated from the open ocean, usually by long, narrow barrier islands or by coral atolls. Lagoons behind barrier islands, which consist mostly of sand, fill gradually with time and become tidal flats, marshes, and swamps. Lagoons of this kind are common along the Atlantic and Gulf coasts of the United States. Lagoons encircled by coral atolls are most common in the South Pacific Ocean.

Small artificial lakes in city parks are sometimes called lagoons.