Geosmithia is a genus of anamorphic fungi of uncertain familial placement in the order Hypocreales.

The genus, circumscribed by Australian mycologist John Pitt in 1979,[1] is widely distributed.

Thousand cankers disease, which affects economically important black walnut (Juglans nigra) populations in North America, is caused by Geosmithia morbida.

[3] Species in the genus are generally similar to those in Penicillium, but can be distinguished from them by forming cylindrical conidia from rough-walled phialides.

However, Geosmithia is a polyphyletic taxon with evolutionary affinities to at least three groups of the euascomycete lineage within the Ascomycota.