Acanthothecis saxicola is a species of saxicolous (rock-dwelling) lichen in the family Graphidaceae. Found in Brazil, it was formally described as a new species in 2022 by André Aptroot, Robert Lücking, and Marcela Eugenia da Silva M.Cáceres. The type specimen was collected near Poço Azul (Riachão, Maranhão) at an altitude of 450m (1,480feet); here, in a cerrado forest, it was found growing on an overhanging sandstone. The lichen has an ochraceous white thallus lacking a cortex and a prothallus. Its asci contains eight spores, and the ascospores are hyaline, measuring 22–30 by 5 μm with 6 to 8 transverse septa. Acanthothecis saxicola contains stictic acid, a lichen product detectable using thin-layer chromatography.