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

[2] The lichen has a somewhat shiny, bullate, olive-green thallus with a cortex but lacking a prothallus; it measures up to 7 cm (2.8 in) in diameter.

The area of the ostioles contains lichexanthone, a lichen product that causes these structures to fluoresce yellow when lit with a long-wavelength UV light.

[2] The characteristics of the lichen that distinguish it from other members of Astrothelium include its distinctly pseudostromatic ascomata with erumpent pseudostromata, with a white cover that contrasts with the surrounding olive-green thallus; and the dimensions of its ascospores (60–70 by 17–20 μm).

[3] Astrothelium simplex is somewhat similar in appearance, but that species does not contain lichexanthone, and it lacks a gelatinous sheath around the ascospores as in A. sinuosum.