The lichen was formally described as a new species in 2004 by lichenologists Marcela Eugenia da Silva Cáceres and Robert Lücking.

[1] Trichothelium angustisporum has a continuous thallus that is pale greenish to yellowish-grey in colour, and can grow up to 5 cm (2 in) across.

The photobiont is Trentepohlia (a genus of green algae), with cells forming short, irregular threads.

It is closely related to and often confused with Trichothelium horridulum, as both species share similar perithecial morphology.

[1] Although T. angustisporum was initially thought to also occur in the southeastern United States (Florida and Louisiana), those populations were described as a distinct species, T. americanum, in 2016.