Orthotrichum casasianum is a species of moss in the family Orthotrichaceae.

It is endemic to the Spanish province of Álava, in the Basque Country.

[2] It grows in less than 200 trees on the banks of the Bayas river.

[3][4] The species is critically endangered, and was added in 2013 to the Basque Catalog of Threatened Species.

[5] This Bryidae-related article is a stub.