Strafford County is a county in the U.S. state of New Hampshire. Its county seat is Dover. Strafford County was one of the five original counties identified for New Hampshire in 1769. Strafford County is included in the Boston-Cambridge-Newton, MA-NH Metropolitan Statistical Area. Strafford County is in southeastern New Hampshire, separated from York County in the state of Maine by the Salmon Falls River. The southern part of the Salmon Falls, from Rollinsford to Dover, is a tidal river that flows into the Piscataqua River. It is the smallest county in New Hampshire by area.