Londonderry is a town in western Rockingham County, New Hampshire, United States. The town is bordered on the north by the city of Manchester and on the east by the town of Derry. The population was 24,129 at the 2010 census. Londonderry is known for its apple orchards and is home to the headquarters of Stonyfield Farm and partial home to Manchester-Boston Regional Airport. The more densely settled portion of town, where 11,037 people resided at the 2010 census, is defined as the Londonderry census-designated place (CDP) and roughly occupies the eastern and southern parts of town, around New Hampshire Route 102.

According to the United States Census Bureau, the town has a total area of 41.9 square miles (108.5 km2), of which 41.8 sq mi (108.3 km2) is land and 0.1 sq mi (0.3 km2) is water, comprising 0.31% of the town. The census-designated place portion of Londonderry has a total area of 12.2 sq mi (32 km2), of which 0.16% is water. Londonderry is drained on the east by Beaver Brook and on the west by Little Cohas Brook and Nesenkeag Brook. The town’s highest point is 535 feet (163 m) above sea level, on “Number Eight Hill” (named after the old school house that used to be on it) north of the center of town. The town is crossed by Interstate 93, New Hampshire Route 102, New Hampshire Route 128, and New Hampshire Route 28.

As of the census of 2000, there were 23,236 people, 7,623 households, and 6,319 families residing in the town. The population density was 555.8 people per square mile (214.6/km²). There were 7,718 housing units at an average density of 184.6 per square mile (71.3/km²).