Astraeus is a genus of fungi in the family Diplocystaceae. The genus, which has a cosmopolitan distribution, contains nine species of earthstar mushrooms. They are distinguished by the outer layer of flesh (exoperidium) that at maturity splits open in a star-shape manner to reveal a round spore sac. Additionally, they have a strongly hygroscopic character—the rays will open when moist, but when hot and dry will close to protect the spore sac. Species of Astraeus grow on the ground in ectomycorrhizal associations with trees and shrubs.