Killingworth is a town in Middlesex County, Connecticut, United States. The town is part of the Lower Connecticut River Valley Planning Region. The population was 6,174 at the 2020 United States Census.

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.