A sclerotium (/skləˈroʊʃəm/; pl.: sclerotia (/skləˈroʊʃə/) is a compact mass of hardened fungal mycelium containing food reserves. One role of sclerotia is to survive environmental extremes. In some higher fungi such as ergot, sclerotia become detached and remain dormant until favorable growth conditions return. Sclerotia initially were mistaken for individual organisms and described as separate species until Louis René Tulasne proved in 1853 that sclerotia are only a stage in the life cycle of some fungi. Further investigation showed that this stage appears in many fungi belonging to many diverse groups. Sclerotia are important in the understanding of the life cycle and reproduction of fungi, as a food source, as medicine (for example, ergotamine), and in agricultural blight management.

Examples of fungi that form sclerotia are ergot (Claviceps purpurea), Polyporus tuberaster, Psilocybe mexicana, Agroathelia delphinii and many species in Sclerotiniaceae. Although not fungal, the plasmodium of slime molds can form sclerotia in adverse environmental conditions.

Sclerotia are often composed of a thick, dense shell with thick and dark cells and a core of thin colorless cells. Sclerotia are rich in hyphae emergency supplies, especially oil. They contain a very small amount of water (5–10%) and can survive in a dry environment for several years without losing the ability to grow. In most cases, the sclerotium consists exclusively of fungal hyphae, whereas some may consist partly of fungal hyphae plexus and partly in between tissues of the substrate (ergot, Sclerotinia). In favorable conditions, sclerotia germinate to form fruiting bodies (basidiomycetes) or mycelium with conidia (in imperfect fungi). Sclerotia sizes can range from a fraction of a millimeter to a few tens of centimeters as, for example Laccocephalum mylittae, which has sclerotia with diameters up to 30 cm and weighing up to 20 kg.^{[verification needed]}

Sclerotia resemble cleistothecia in both their morphology and the genetic control of their development. This suggests the two structures may be homologous, sclerotia being vestigial cleistothecia that lost the capacity to produce ascospores.

In the Middle Ages, Claviceps purpurea sclerotia-contaminated rye grain used in bread led to ergotism, by way of which thousands of people were killed and mutilated. The sclerotia contain alkaloids that cause paranoia and hallucinations, twitches, spasms, loss of peripheral sensation, edema and gangrene.

Louis René Tulasne discovered the relationship between infected rye plants and ergotism in the 19th century. With this discovery, more efforts were developed to reduce sclerotia from growing on rye and ergotism became rare. However, in 1879–1881 an outbreak developed in Germany, in 1926–1927 Russia was infected, and in 1977–1978 Ethiopia was infected.

Pleurotus tuber-regium, which forms edible sclerotia up to 30 cm wide, has a history of economic importance in Africa as food and as a medicinal mushroom.

For example, Claviceps purpurea sclerotia form and begin regrowth in the spring, infecting grass and rye plants by way of releasing their ascospores from perithecia. Claviceps purpurea can infect a wide variety of plants by infecting the ovaries. The fungal spores germinate at the anthesis and grow down the pollen tube without branching any hyphae outward. When the fungus reaches the bottom of the ovary, it leaves the pollen tube path and enters the vascular tissues where it branches its hypha. Approximately seven days into the infection, the mycelium produces conidia. The conidia are then secreted out of the plant in a sugary liquid that insects, attracted by the sugars, transfer to other plants. After two weeks of being infected by the fungus, the plant no longer generates the sugary liquid, and the fungus produces sclerotia. The sclerotium is an overwinter structure, which contains ergot alkaloids. Claviceps purpurea's life cycle is an interesting model for plant pathologists and cell biologists because: