Deep River (formerly known as Saybrook) is a town in Middlesex County, Connecticut, United States. The population was 4,629 at the 2010 census. The town center is also designated by the U.S. Census Bureau as a census-designated place (CDP). Deep River is part of what the locals call the "Tri-town Area" made up of Deep River, Chester, and Essex, Connecticut. Every year on the third Saturday in July, Deep River hosts the Deep River Ancient Muster.