Placidium nigrum is a species of squamulose (scaley), ground-dwelling lichen in the family Verrucariaceae.

It is found in Northwest China and the Tibetan Plateau, where it grows on sandy soil surfaces in semi-arid and arid regions.

The species epithet nigrum alludes to the black area surrounding the thallus that results from an aggregation of pycnidia (asexual fruiting bodies).

These abundant, tiny pycnidia occur both superficially on the thallus surface (laminal) and on the lobe edges (marginal).

Placidium nigrum does not react with any of the standard chemical spot tests, and no lichen products were detected from the species using thin-layer chromatography.