Allenstown is a town in Merrimack County, New Hampshire, United States. The population was 4,322 at the 2010 census. Allenstown includes a portion of the village of Suncook. Just over one-half of the town's area is covered by Bear Brook State Park.

Allenstown takes its name from 17th-century provincial governor Samuel Allen. It was granted in 1721 but not incorporated until 2 July 1831. A part of neighboring Bow was annexed to Allenstown in 1815, and a portion of Hooksett was annexed in 1853.

Most of the town's earliest settlement occurred in the eastern part of town along Deerfield Road, around the area now mostly occupied by Bear Brook State Park. Following the Civil War, the town's population shifted from the east to the west part of town, centered around the confluence of the Merrimack and Suncook Rivers, in an area now known as Suncook.

Railroads were instrumental to the development of Allenstown. First, a branch of the Concord and Portsmouth Railroad running to Hooksett arrived in the late 1850s, followed by the Suncook Valley Railroad in 1869, which first ran to Pittsfield and later, to Center Barnstead. Two railroad stations existed in Allenstown: one in Suncook village, along what is now Canal Street, and the other in the northern part of town, along what is now Verville Road.

Allenstown, at the junction of the Suncook and Merrimack Rivers, proved a prime location in which to harness the rivers' power for manufacturing. The China Mill, the only large textile mill built in the Allenstown part of Suncook, was built in 1868. At this time, a large amount of French Canadians, mostly from Quebec, began emigrating to the area to work in the mills. Eventually, Suncook became one of many New England industrial villages known to locals as "le petit Canada."

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