Fenwick is a borough in, in the town of Old Saybrook. The population was 52 at the 2000 census, making it the least populous municipality in Connecticut.

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.

The Fenwick Historic District covers an area of approximately 195 acres and was added to the National Register of Historic Places in 1995. It includes 60 buildings in the center of Fenwick, as well as the Fenwick Golf Course.

The District is composed largely of Shingle-style residences from early in the century. The Historic District comprises the bulk of the community, however some residences were built in the 1950s in a more modern style.

In the late 1800s The Fenwick Golf Course was created from several empty lots in the center of the district and is the site of the Stephen Potter Cup.

As of the of 2000 census, there were 52 people, 26 households, and 17 families residing in the borough. The racial makeup of the borough was 100.00% White.

 Fenwick was the lifelong home to actress Katharine Hepburn until she died on June 29, 2003 at the age of 96. Actress Rosemary Murphy, New York City mayor John Lindsay, and several members of the Whitney family have also lived there at one time or another. Fenwick is also home to Christopher Rehm, Conor Millard and Charlie McGrath.