Moriches Bay (/moʊˈrɪtʃɪz/ ⓘ moh-RITCH-iz)[1] is a lagoon system on the south shore of Long Island, New York.

[3] This bayside habitat includes the tidal creeks and marshes feeding into Moriches Bay from the Long Island mainland.

[4][5] Moriches Bay is a natural habitat for shellfish, migrating and wintering waterfowl, colonial nesting waterbirds, beach-nesting birds, migratory shorebirds, raptors, and rare plants.

It has been repeatedly adjudicated (even to the U.S. Supreme Court) that the land grants in the Dongan (Governor of New York) patents (Islip, Brookhaven, Southampton and East Hampton Towns, 1686) are valid.

However, since 1968 the federal government has been attempting to take title of and lay claim to, by adverse possession, the bay bottoms of Islip and Brookhaven towns extending outward from the barrier beach.