Gymnopilus thiersii is a species of mushroom in the Cortinariaceae family. It is nearly identical to Gymnopilus luteofolius, but this species differs in having different context colors and dextrinoid spores. The type location for Gymnopilus thiersii is Wunderlich Park in San Mateo County, California. This species contains the hallucinogenic drugs psilocybin and psilocin, and stains blue-green where damaged.