Botulinum toxin, or botulinum neurotoxin (BoNT), is a neurotoxic protein produced by the bacterium Clostridium botulinum and related species. It prevents the release of the neurotransmitter acetylcholine from axon endings at the neuromuscular junction, thus causing flaccid paralysis. The toxin causes the disease botulism. The toxin is also used commercially for medical and cosmetic purposes.

The seven main types of botulinum toxin are named types A to G (A, B, C1, C2, D, E, F and G). New types are occasionally found. Types A and B are capable of causing disease in humans, and are also used commercially and medically. Types C–G are less common; types E and F can cause disease in humans, while the other types cause disease in other animals.

Botulinum toxins are among the most potent toxins known. Intoxication can occur naturally as a result of either wound or intestinal infection or by ingesting formed toxin in food. The estimated human lethal dose of type A toxin is 1.3–2.1 ng/kg intravenously or intramuscularly, 10–13 ng/kg when inhaled, or 1000 ng/kg when taken by mouth. Commercial forms are marketed under the brand names Botox (onabotulinumtoxinA), Dysport/Azzalure (abobotulinumtoxinA), Letybo (letibotulinumtoxinA), Myobloc (rimabotulinumtoxinB), Xeomin/Bocouture (incobotulinumtoxinA), and Jeuveau (prabotulinumtoxinA).