Psilocybe allenii is a species of agaric fungus in the family Hymenogastraceae. The fruitbodies (mushrooms) grow on rotting wood, especially wood chips used in garden landscaping. The caps of the mushrooms are brown to buff, broadly convex to flattened and have a diameter up to 9 cm (3.5 in), while the white stipes are up to 9 cm (3.5 in) long and 0.7 cm (0.3 in) thick. As a bluing species in the genus Psilocybe, P. allenii contains the psychoactive compounds psilocin and psilocybin, and it is consumed recreationally for its hallucinogenic properties. It is closely related to Psilocybe cyanescens, from which it differs macroscopically by the lack of a wavy cap margin.