Somersworth is a city in Strafford County, New Hampshire, United States. The population was 11,766 at the 2010 census. Somersworth has the smallest area and third-lowest population of New Hampshire's 13 cities.
Somersworth, originally called Sligo after Sligo in Ireland, was settled before 1700 as a part of Dover. It was organized in 1729 as the parish of Summersworth, meaning summer town, because during that season the ministers would preach here. It was set off and incorporated in 1754 by Colonial Governor Benning Wentworth, and until 1849 included Rollinsford. A clerical error at incorporation contracted the name to Somersworth. It would be incorporated as a city in 1893, before which it was also known as Great Falls.
Situated where the Salmon Falls River drops 100 feet (30 m) over a mile, Somersworth early became a mill town, beginning with gristmills and sawmills. In 1822, the brothers Isaac and Jacob Wendell of Boston purchased for $5,000 a gristmill with its water rights at the Great Falls. They established the Great Falls Manufacturing Company, a textile business that expanded to include three mills for spinning thread and weaving cotton and woolen fabrics, specializing in "drillings, shirtings and sheetings." Throughout the 19th century, other expansive brick mill buildings, including a Bleachery and Dye Works, were erected beside the river. A gate house at the dam directed water as needed, regulating the flow either into the river or a company canal, which itself had gates sending it under the mill. Water power turned the wheels and belts that operated mill machinery. The railroad arrived in the early 1840s, before which goods were carted to Dover.