Amanita muscaria, commonly known as the fly agaric or fly amanita, is a basidiomycete fungus of the genus Amanita. It is a large white-gilled, white-spotted mushroom typically featuring a bright red cap covered with distinctive white warts. It is one of the most recognisable fungi in the world.

A. muscaria exhibits complex genetic diversity that suggests it is a species complex rather than a single species. It is a widely distributed mushroom native to temperate and boreal forests of the Northern Hemisphere, now also naturalised in the Southern Hemisphere, forming symbiotic relationships with various trees and spreading invasively in some regions.

Its name derives from its traditional use as an insecticide. It can cause poisoning, especially in children and those seeking its hallucinogenic effects, due to psychoactive compounds like muscimol and the ibotenic acid; however, fatal poisonings are extremely rare. Boiling it reduces toxicity by removing water-soluble ibotenic acid into the discarded water. Drying converts ibotenic acid into muscimol, lowering toxicity but retaining psychoactive effects. Some cultures use it as food after preparation. Indigenous peoples of Siberia used A. muscaria as an inebriant and entheogen. It has been controversially linked to Santa Claus, Viking berserkers, Vedic soma, and early Christianity, though evidence is sparse and disputed. Its rise in the 2020s as a legal hallucinogen alternative has led to Food and Drug Administration scrutiny.

A. muscaria has appeared in art and literature since the Renaissance, becoming iconic in fairy tales, children’s books, and media like the Super Mario games and Disney’s Fantasia. It has also influenced literary depictions of altered perception—most notably in Alice’s Adventures in Wonderland—and has been referenced in novels by writers including Oliver Goldsmith, Thomas Pynchon, and Alan Garner.

The name of the mushroom in many European languages is thought to derive from its use as an insecticide when sprinkled in milk. This practice has been recorded from Germanic- and Slavic-speaking parts of Europe, as well as the Vosges region and pockets elsewhere in France, and Romania. Albertus Magnus was the first to record it in his work De vegetabilibus some time before 1256, commenting "vocatur fungus muscarum, eo quod in lacte pulverizatus interficit muscas" ("it is called the fly mushroom because it is powdered in milk to kill flies").

The 16th-century Flemish botanist Carolus Clusius traced the practice of sprinkling it into milk to Frankfurt in Germany, while Carl Linnaeus, the "father of taxonomy", reported it from Småland in southern Sweden, where he had lived as a child. He described it in volume two of his Species Plantarum in 1753, giving it the name Agaricus muscarius, the specific epithet deriving from Latin musca meaning "fly". It gained its current name in 1783, when placed in the genus Amanita by Jean-Baptiste Lamarck, a name sanctioned in 1821 by the "father of mycology", Swedish naturalist Elias Magnus Fries. The starting date for all the mycota had been set by general agreement as January 1, 1821, the date of Fries's work, and so the full name was then Amanita muscaria (L.:Fr.) Hook. The 1987 edition of the International Code of Botanical Nomenclature changed the rules on the starting date and primary work for names of fungi, and names can now be considered valid as far back as May 1, 1753, the date of publication of Linnaeus's work. Hence, Linnaeus and Lamarck are now taken as the namers of Amanita muscaria (L.) Lam..

The English mycologist John Ramsbottom reported that Amanita muscaria was used for getting rid of bugs in England and Sweden, and bug agaric was an old alternative name for the species. French mycologist Pierre Bulliard reported having tried without success to replicate its fly-killing properties in his work Histoire des plantes vénéneuses et suspectes de la France (1784), and proposed a new binomial name Agaricus pseudo-aurantiacus because of this. One compound isolated from the fungus is 1,3-diolein (1,3-di(cis-9-octadecenoyl)glycerol), which attracts insects. It has been hypothesised that the flies intentionally seek out the fly agaric for its intoxicating properties. An alternative derivation proposes that the term fly- refers not to insects as such but rather the delirium resulting from consumption of the fungus. This is based on the medieval belief that flies could enter a person's head and cause mental illness. Several regional names appear to be linked with this connotation, meaning the "mad" or "fool's" version of the highly regarded edible mushroom Amanita caesarea. Hence there is oriol foll "mad oriol" in Catalan, mujolo folo from Toulouse, concourlo fouolo from the Aveyron department in Southern France, ovolo matto from Trentino in Italy. A local dialect name in Fribourg in Switzerland is tsapi de diablhou, which translates as "Devil's hat".

Amanita muscaria is the type species of the genus. By extension, it is also the type species of Amanita subgenus Amanita, as well as section Amanita within this subgenus. Amanita subgenus Amanita includes all Amanita with inamyloid spores. Amanita section Amanita includes the species with patchy universal veil remnants, including a volva that is reduced to a series of concentric rings, and the veil remnants on the cap to a series of patches or warts. Most species in this group also have a bulbous base. Amanita section Amanita consists of A. muscaria and its close relatives, including A. pantherina (the panther cap), A. gemmata, A. farinosa, and A. xanthocephala. Modern fungal taxonomists have classified Amanita muscaria and its allies this way based on gross morphology and spore inamyloidy. Two recent molecular phylogenetic studies have confirmed this classification as natural.