Loudon is a town in Merrimack County, New Hampshire, United States. The population was 5,576 at the 2020 census. Loudon is the home of New Hampshire Motor Speedway. The main village in town, where 711 people lived at the 2020 census, is defined as the Loudon census-designated place and is located along the Soucook River at the southern terminus of New Hampshire Route 129.