Astrothelium xanthosordithecium is a species of corticolous (bark-dwelling, crustose lichen in the family Trypetheliaceae.

The species epithet xanthosordithecium alludes to its distinctive feature: the presence of lichexanthone, a yellow pigment, contrasting with the brownish hamathecium (a tissue in the ascomata containing spore-producing cells) inspersion of A. sordithecium.

[2] The thallus of Astrothelium xanthosordithecium is slightly shiny and olivaceous green, spreading up to 110 cm (43 in) in diameter and about 0.1 mm thick.

The lichen produces eight spores per ascus, which are clear (hyaline), dividided into four compartments by septa (3-septate), and measure 34–37 by 10–12 μm.

Thin-layer chromatography confirmed the presence of lichexanthone, a secondary metabolite that contributes to the lichen's unique characteristics.