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.
Fenwick is set off from the town center of Old Saybrook by a large cove over a causeway. It is located exactly where the Connecticut River flows into Long Island Sound. Fenwick has two lighthouses, the Inner and the Outer. There is also a single private beach about a quarter of a mile away from the lighthouse. The Inner is at the tip of Lynde Point, Fenwick's peninsula, and the Outer is a quarter mile off shore, connected by a rough jetty. The Outer Light is the lighthouse shown on many Connecticut license plates.