Cordyceps is a genus of ascomycete fungi (sac fungi) that includes about 400 described species. All Cordyceps species are endoparasitoids, mainly on insects and other arthropods (they are thus entomopathogenic fungi); a few are parasitic on other fungi. The best known species of the genus is Cordyceps sinensis, first recorded as yartsa gunbu in Tibet in the 15th Century. It is known as yarsha gumba in Nepal. The Latin etymology describes cord as club, ceps as head, and sinensis as Chinese. Cordyceps sinensis, known in English commonly as caterpillar fungus, is considered a medicinal mushroom in oriental medicines, such as traditional Chinese medicines and traditional Tibetan medicine.

Cordyceps species are sources of biochemicals with interesting biological and pharmacological properties, like cordycepin; the anamorph of Cordyceps subsessilis (Tolypocladium inflatum) was the source of ciclosporin—a drug helpful in human organ transplants.