Manchester is the most populous city in the U.S. state of New Hampshire. Located on the banks of the Merrimack River, it had a population of 115,644 at the 2020 census. Manchester is the tenth-most populous city in New England. Along with the city of Nashua, it is one of two seats of New Hampshire's most populous county, Hillsborough County. The Manchester–Nashua metropolitan area has approximately 423,000 residents and lies near the northern end of the Northeast megalopolis.

Manchester was first named by the merchant and inventor Samuel Blodget(t), eponym of Samuel Blodget Park and Blodget Street in the city's North End. His vision was to create a great industrial center similar to that of the original Manchester in England, which was the world's first industrialized city. During the Industrial Revolution, Manchester was a major industrial and economic hub for New England, with the Amoskeag Manufacturing Company being the largest cotton textile plant in the world. After World War II, many textile manufacturing jobs left Manchester, but new industries and companies were introduced to the city, such as DEKA.

The native Pennacook people called Amoskeag Falls on the Merrimack River—the area that became the heart of Manchester—Namaoskeag, meaning "good fishing place". In 1722, John Goffe III settled beside Cohas Brook, later building a dam and sawmill at what was dubbed "Old Harry's Town". It was granted by Massachusetts in 1727 as "Tyngstown" to veterans of Queen Anne's War who served in 1703 under Captain William Tyng. But at New Hampshire's 1741 separation from Massachusetts, the grant was ruled invalid and substituted with Wilton, Maine, resulting in a 1751 rechartering by Governor Benning Wentworth as "Derryfield"—a name that lives on in Derryfield Park, Derryfield Country Club, and the private Derryfield School.

In 1807, Samuel Blodget opened a canal and lock system to allow vessels passage around the falls, part of a network developing to link the area with Boston. He envisioned a great industrial center arising, "the Manchester of America", in reference to Manchester, England, then at the forefront of the Industrial Revolution. In 1809, Benjamin Prichard and others built a water-powered cotton spinning mill on the western bank of the Merrimack. Apparently following Blodgett's suggestion, Derryfield was renamed "Manchester" in 1810, the year the mill was incorporated as the Amoskeag Cotton & Woolen Manufacturing Company. It would be purchased in 1825 by entrepreneurs from Massachusetts, expanded to three mills in 1826, and then incorporated in 1831 as the Amoskeag Manufacturing Company.

Amoskeag engineers and architects planned a model company town on the eastern bank, founded in 1838 with Elm Street as its main thoroughfare. Incorporation as a city followed for Manchester in 1846, soon home to the largest cotton mill in the world—Mill No. 11, stretching 900 feet (270 m) long by 103 feet (31 m) wide, and containing 4,000 looms. Other products made in the community included shoes, cigars, and paper. The Amoskeag foundry made rifles, sewing machines, textile machinery, fire engines, and locomotives in a division called the Amoskeag Locomotive Works (later, the Manchester Locomotive Works). The rapid growth of the mills demanded a large influx of workers, resulting in a flood of immigrants, particularly French Canadians. Many current residents descend from these workers. In 1871, the arch dam was built on the Merrimack River, enhancing the mill's water power delivery system. By 1912, the production of woven cloth in the Millyard had reached a production rate of 50 miles in length per hour.

Throughout the late 19th century and the early 20th century, the city began to expand outward, and many streetcar suburbs such as Mast Road were built. Manchester was formerly home to a streetcar network, the Manchester Street Railway. The streetcar network was replaced with a bus network in the 1940s.

In 1922, 17,000 workers from two of the city's largest companies (Amoskeag and Stark Manufacturing Companies) went on strike for a period of nine months. After the strike, the textile industry began a slow decline, with the Great Depression hitting the city particularly hard. The Amoskeag Manufacturing Company declared bankruptcy in 1935. During the Great Flood of 1936, the McGregor Bridge was destroyed and $2.5 million of damage was incurred to the city's mills and buildings. After the flood, the Amoskeag Manufacturing Company reorganized out of bankruptcy as Amoskeag Industries, diversifying its manufacturing operations with new industries in the Millyard.

Manchester's economy benefitted from World War II, as the city was already well-positioned and equipped with industry to handle war-time production. In 1941, Grenier Field, on the city's border with Londonderry, was converted to a U.S. Army air base.