Fenwick is a borough in Middlesex County, Connecticut, United States, in the town of Old Saybrook. The borough is part of the Lower Connecticut River Valley Planning Region. The population was 53 at the 2020 census, making it the least populous borough in Connecticut. Most of the borough is included in the Fenwick Historic District, listed on the National Register of Historic Places. In 1995, the district included 66 contributing buildings and one other contributing site.

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.