The Tabusintac Lagoon and River Estuary is a wetland in Alnwick Parish, Northumberland County, in north-eastern New Brunswick, Canada. It was classified as a wetland of international importance on June 10, 1993. It is also a globally significant Important Bird Area for the population of common terns, and shorebirds in general, that it supports. Primarily a shallow coastal estuary with gentle slopes, the 50 km² site is underlain by various sedimentary rocks, including red sandstone and shale. The lagoon system is protected from the Gulf of Saint Lawrence by a constantly shifting barrier beach and dune system that frequently blocks commonly used navigation channels.