York County is the southwesternmost county in the U.S. state of Maine, along the state of New Hampshire's eastern border. It is divided from Strafford County, New Hampshire, by the Salmon Falls River and the connected tidal estuary, the Piscataqua River. York County was permanently established in 1639. Several of Maine's earliest colonial settlements are found in the county, which is the state's oldest and one of the oldest in the United States. As of the 2020 census, its population was 211,972, making it Maine's second-most populous county. Its county seat is Alfred. York County is part of the Portland–South Portland, Maine Metropolitan Statistical Area.

Strafford County is a county in the U.S. state of New Hampshire. As of the 2020 census, the population was 130,889. Its county seat is Dover. Strafford County was one of the five original counties identified for New Hampshire in 1769. It was named after William Wentworth, 2nd Earl of Strafford in the mistaken belief that he was the ancestor of governor John Wentworth – although they were distantly related, William had no descendants. The county was organized at Dover in 1771. In 1840, the size of the original county was reduced with the creation of Belknap County.