Claviceps purpurea is an ergot fungus that grows on the ears of rye and related cereal and forage plants. Consumption of grains or seeds contaminated with the survival structure of this fungus, the ergot sclerotium, can cause ergotism in humans and other mammals. C. purpurea most commonly affects outcrossing species such as rye, as well as triticale, wheat and barley. It affects oats only rarely.

Ergot or ergot fungi refers to a group of fungi of the genus Claviceps.