Trichothelium longisetum is a little-known species of wood-dwelling, crustose lichen in the family Trichotheliaceae. Found in northeastern Ecuador, it was formally described as a new species by lichenologists Patrick M. McCarthy and Zdeněk Palice in 2003. The species epithet longisetum refers to the long, narrow that characterize this species.
The thallus of Trichothelium longisetum is crustose, thin, and greyish-brown in colour, with a (green algal) that does not form radiating plates. The are numerous, superficial, more or less spherical, and black, measuring 0.52 to 1.58 mm in diameter. These perithecia are notable for their long, narrow, and numerous black setae.
The are colourless, elongate to with a length of 42 to 64 μm, and contain multiple septa. (structures bearing), have not been observed in this species.
Trichothelium longisetum has some similarities to the foliicolous Trichothelium ulei, but can be distinguished from this species by its larger ascospores that have more septa.
Trichothelium longisetum has only been found in Napo Province, Ecuador. The type specimen was collected by the second author from Yasuní National Park at an altitude of 300m (1,000feet), where it was growing on the wood of a tree stump in a rainforest. This habitat is unique among the predominantly, or leaf-dwelling, species in the genus Trichothelium. The species' occurrence on wood sets it apart from its closest relatives, which are typically found on leaves.