Phacopsis thallicola is a species of lichenicolous (lichen-dwelling) fungus in the family Parmeliaceae. It was first formally described as a new species in 1852 by Italian botanist Abramo Bartolommeo Massalongo, as Lecidea thallicola. The type specimen, collected from the province of Treviso in Italy, was growing on the foliose lichen Parmelia caperata (now known as Flavoparmelia caperata). Dagmar Triebel and Gerhard Walter Rambold transferred the taxon to the genus Phacopsis in 1988. The known generic hosts of Phacopsis thallicola are all in the Parmeliaceae: Parmotrema, Cetrelia, Flavopunctelia, and Hypotrachyna.