Longdale Furnace is an unincorporated community located east of Clifton Forge in Alleghany County, Virginia, United States. Much of the community was designated a national historic district and added to the National Register of Historic Places in 1998. The district encompasses 27 contributing buildings, 8 contributing sites, and 3 contributing structures historically associated with the Longdale Iron Company foundries' operations and personnel. It includes the original stone furnace stack (1827), two-story Longdale Iron Company Office (late-1820s), Longdale Iron Company Laboratory, two tall cylindrical brick chimneys, brick machine shop, Firmstone-Johnson House (1870s–1880s), the Queen Anne-style Assistant Manager's House (1870s–1880s), and the Company Doctor's Office.