Trichothelium longisetum is a little-known species of wood-dwelling, crustose lichen in the family Porinaceae.

Found in northeastern Ecuador, it was formally described as a new species by lichenologists Patrick M. McCarthy and Zdeněk Palice in 2003.

The thallus of Trichothelium longisetum is crustose, thin, and greyish-brown in colour, with a trentepohlioid (green algal) photobiont that does not form radiating plates.

The type specimen was collected by the second author from Yasuní National Park at an altitude of 300 m (980 ft), where it was growing on the wood of a tree stump in a rainforest.

The species' occurrence on wood sets it apart from its closest relatives, which are typically found on leaves.