The Ostropomycetidae are a subclass of mostly lichen-forming fungi in the class Lecanoromycetes. The subclass was circumscribed in 2004 by Catherine Reeb, François M. Lutzoni, and Claude Roux. It contains eight orders and 36 families. Arctomiaceae is the only family in the Ostropomycetidae that associates with cyanobacteria of the order Nostocales as its main photobiont partner.