Astrothelium simplex was formally described as a new species by the lichenologists André Aptroot and Suzana Maria de Azevedo Martins in 2016.

[2] The thallus of Astrothelium simplex is corticate, bullate, somewhat shiny, and continuous, covering areas up to 7 cm in diameter and approximately 0.3 mm thick.

The ascomata are pyriform (pear-shaped), measuring about 0.6–0.9 mm in diameter, typically aggregated in groups of 2–5, and mostly immersed in the bark tissue.

Ascospores are hyaline, 3-septate, fusiform, measuring 56–63 by 15–19 μm, with pointed ends and diamond-shaped lumina, and lack a gelatinous sheath.

[2] This species is found on the smooth bark of trees in the Atlantic Forest and at the time of its original publication was known to occur only in Brazil.