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. .
Fenwick is set off from the town center of Old Saybrook by a large cove crossed by a causeway. It is located exactly where the Connecticut River flows into Long Island Sound, and sits on the river's west side. The town has two lighthouses, the Inner and the Outer. 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.