Fohnsdorf, city, southeast-central Austria, near the Mur River, west of Knittelfeld. Fohnsdorf was first mentioned in 1141 as the site of a fortress belonging to the archbishops of Salzburg and was a coal-mining (lignite) centre from 1670 to the late 20th century. Notable landmarks are the Romanesque St. Rupert’s Church and the ruined fortress (1309). Fohnsdorf is a market and service centre for the rural hinterland. Pop. (2008) 8,114.