In the U.S. state of Connecticut, U.S. Route 1 is a major east–west state highway along Long Island Sound. It has been replaced by Interstate 95 as a through route, which it closely parallels, and now primarily serves as a local business route. Despite its east–west orientation, it is part of a north–south route, and so most signs show it as north–south.

US 1 in Connecticut stays close to Interstate 95 throughout the state and has many junctions with it. It goes through the following towns as the roads listed below. From Greenwich to Branford, US 1 is mostly a 4-lane or 6-lane principal arterial road (with some 2-lane sections in dense areas). From Branford to Stonington, US 1 becomes a 2-lane or 4-lane minor arterial road (it is classified as a principal arterial road within Waterford town). One note of interest is that Route 1 in Connecticut has almost a direct east–west route. Because of this, in some places, signs may say Route 1 east or west, and only a short distance away, another sign may say Route 1 north or south. This can make it confusing for those who are not familiar with the area.[original research?] Most of US 1 through Connecticut encompasses its predecessor, the Boston Post Road, and much of it is still locally named Boston Post Road.

US 1 generally follows the old Boston Post Road and turnpike roads built to replace it. Specifically, the Greenwich Road, chartered in 1792, was the part in Greenwich, and became part of the longer Connecticut Turnpike (New York to Fairfield) in 1806. The New Haven and Milford Turnpike (1802) continued from Milford to New Haven, and the New London and Lyme Turnpike (1807) connected Old Lyme with New London.