Deep River is a town in Middlesex County, Connecticut. The town is part of the Lower Connecticut River Valley Planning Region. The population was 4,415 at the 2020 census. The town center is 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 the towns of Deep River, Chester, and Essex.

Middlesex County is a county in the south central part of the U.S. state of Connecticut. As of the 2020 census, the population was 164,245. The county was created in May 1785 from portions of Hartford County and New London County.