Fenwick is a borough in Middlesex County, Connecticut, United States, in the town of Old Saybrook. The population was 52 at the 2000 census, making it the least populous municipality in Connecticut. It is a popular summer colony. Most of the borough is included in Fenwick Historic District, a historic district listed on the National Register of Historic Places. In 1995, the district included 66 contributing buildings and one other contributing site.