T. ludificans T. luteominia T. nashii T. rosei Tomnashia is a genus of lichen-forming fungi in the family Teloschistaceae.

It has four species of saxicolous (rock-dwelling), crustose lichens that occur in southwestern North America.

[1] Tomnashia was circumscribed in 2017 by lichenologists Sergey Kondratyuk and Jae-Seoun Hur, who bestowed upon the taxon an eponym in honour of Thomas Hawkes Nash III, a significant contributor to lichenology, especially concerning North American lichen flora.

[2] Through combined phylogenetic analysis, it was determined that Tomnashia resides in the outermost position among monophyletic groups of the Polycauliona subclade of the subfamily Xanthorioideae.

Other genera in the same subfamily, such as Massjukiella and Verrucoplaca, have a much broader distribution in the Northern Hemisphere or worldwide.