A new species of the genus Aethiophenax Mahunka, 1981 (Acari: Prostigmata: Acarophenacidae), A. mycetophagi Arjomandi and Hajiqanbar sp. nov., is described and illustrated based on adult females. Mites were collected from the beneath elytra of hairy fungus beetles, Mycetophagus quadripustulatus (Linnaeus, 1761) (Col.: Tenebrionoidea: Mycetophagidae) grazing on oyster mushrooms on a bough of a tree in Mazandaran province, northern Iran. The new species, as fourth representative of the genus, is characterized by all dorsal setae thin and relatively long (26–35) in comparison with those of congeners; all ventral setae thin, setae 2a longest and 1b shortest of venter of idiosoma; tibiotarsus I with solenidion ϕ slightly clavate and as long as ω; tarsi II with seta pl″ modified and spine-like; genu III with two setae (l′ and v′). A key to the world species of the genus is also provided.