Lithothelium grossum is a species of corticolous (bark-dwelling) lichen in the family Pyrenulaceae.

Found in China, it was formally described as a new species in 2006 by Dutch lichenologist André Aptroot.

The type specimen was collected in the Xishuangbanna Tropical Botanical Garden (Mengla County, Yunnan) at an altitude of 550 m (1,800 ft); here it was found growing on a tree trunk.

Lithothelium grossum is characterized by its large hyaline (translucent) ascospores, which measure 27–37 by 12–17 μm; these are the largest of hyaline-spored species in genus Lithothelium.

This Eurotiomycetes-related article is a stub.