Geosmithia is a genus of anamorphic fungi of uncertain familial placement in the order Eurotiales. The genus, circumscribed by Australian mycologist John Pitt in 1979, is widely distributed. A 2008 estimate placed ten species in the genus, but several new species have since been described. Thousand cankers disease, which affects economically important black walnut (Juglans nigra) populations in North America, is caused by Geosmithia morbida.