Placidium is a genus of crustose to squamulose to almost foliose lichens.

[2] Members of the genus are commonly called stiplescale lichens or earthscale.

[2]: 327 The genus was circumscribed in 1855 by Italian lichenologist Abramo Bartolommeo Massalongo.

[3] María Prieto and Ibai Olariaga suggest that Placidium is a superfluous illegitimate name according to strict application of nomenclatural rules.

When Othmar Breuss resurrected the genus for use in 1996,[4] he assigned P. michelii as its type, even though Massalongo did not include P. michelii in Placidium in his original 1855 circumscription, but rather in a later treatment of the genus.