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. Middlesex County is included in the Hartford-East Hartford-Middletown metropolitan statistical area known as Greater Hartford. As with all eight of Connecticut's counties, there is now no county government and no county seat.