Ferdinand Theissen (27 July 1877 in Krefeld – 5 September 1919) was a German-Austrian Jesuit priest and mycologist. He studied theology at the seminary in Feldkirch, then from 1902 to 1908 was stationed in São Leopoldo, Brazil. Following his return to Europe he continued his studies in Valkenburg and Innsbruck, and in 1914 returned to Feldkirch as a schoolteacher. He died in September 1919 as a result of a climbing accident during a collection excursion in the Vorarlberg Alps.