Phacopsis lethariellae is a species of lichenicolous (lichen-dwelling) fungus in the family Parmeliaceae. It was formally described as a new species in 1995 by Josef Hafellner and Gerhard Rambold. The type specimen was collected by the first author from La Fortaleza (La Gomera) at an altitude of , where it was found growing on the thallus of the lichen Lethariella intricata. It causes formations of galls, which also creates a torsion on the thallus. It has dark brown to black apothecia that are typically 0.3âÂÂ0.5 mm in diameter, with a convex . Its ascospores are ellipsoid to ovoid, measuring 11âÂÂ13 by 6âÂÂ7 micrometres. The fungus, known to occur only in the Canary Islands, is named after the genus of its host.