alpha-Amanitin or α-amanitin is a cyclic nonribosomal peptide of eight amino acids. It is one of the toxins found in the Amanita genus of mushrooms that makes them so deadly. It is also found in the unrelated mushroom Galerina autumnalis.

Particularly dangerous about this toxin is its delayed effects. It slowly attacks an RNA polymerase in the liver. Few effects are reported within 10 hours; it is not unusual for significant effects to take as much as 24 hours after ingestion. This is far past the time when the stomach could be safely pumped. Diarrhea and cramps are the first symptom, but those pass, leaving a false remission. Then, on the 4th to 5th day, the toxin starts to have severe effects on the liver and kidneys, leading to total system failure in both. Death usually takes place around a week from ingestion.

Around 15% of those poisoned will die in around 10 days progressing through a comatose stage to renal failure, liver failure, hepatic coma, respiratory failure and death. Those who recover are unlikely to regain full health. Early diagnosis can be tricky and treatment is mainly supportive (gastric lavage, activated charcoal, fluid resuscitation) but includes various drugs to counter the amatoxins, including penicillin and cephalosporin derivatives, and can extend to an orthotopic liver transplant. The most reliable method to treat amanitin poisoning is through having the stomach pumped immediately after ingestion; however, the onset of symptoms is generally too late for this to be an option.