Amanita phalloides (/æməˈnaɪtə fəˈlɔɪdiːz/), commonly known as the death cap, is a deadly poisonous basidiomycete fungus, one of many in the genus Amanita. Widely distributed across Europe, but now sprouting in other parts of the world, A. phalloides forms ectomycorrhizas with various broadleaved trees. In some cases, the death cap has been introduced to new regions with the cultivation of non-native species of oak, chestnut, and pine. The large fruiting bodies (mushrooms) appear in summer and autumn; the caps are generally greenish in colour with a white stipe and gills. The cap colour is variable, including white forms, and is thus not a reliable identifier.

These toxic mushrooms resemble several edible species (most notably Caesar's mushroom and the straw mushroom) commonly consumed by humans, increasing the risk of accidental poisoning. Amatoxins, the class of toxins found in these mushrooms, are thermostable: they resist changes due to heat, so their toxic effects are not reduced by cooking.

A. phalloides is one of the most poisonous of all known mushrooms. It is estimated that as little as half a mushroom contains enough toxin to kill an adult human. It has been involved in the majority of human deaths from mushroom poisoning,[1] possibly including Roman Emperor Claudius in AD 54 and Holy Roman Emperor Charles VI in 1740. It has been the subject of much research and many of its biologically active agents have been isolated. The principal toxic constituent is α-Amanitin, which causes liver and kidney failure.