Loudon is a town in Merrimack County, New Hampshire, United States. The population was 5,317 at the 2010 census. Loudon is the home of New Hampshire Motor Speedway.The primary settlement in town, where 559 people resided at the 2010 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.