Botulin toxin or botox is the toxic compound produced by the bacterium Clostridium botulinum. It is an enzyme that breaks down one of the fusion proteins that allow neurons to release acetylcholine at a neuromuscular junction. By interfering with nerve impulses in this way, it causes paralysis of muscles in botulism. The toxin itself is a two-chain polypeptide with a 100-kd heavy chain joined by a disulfide bond to a 50-kd light chain.

Botulin toxin is used (usually under a trademarked name such as "Botox") for producing long-term (months) paralysis of muscles. This was intended for the relief of uncontrollable muscle spasm, but is increasingly being used for cosmetic purposes, to paralyse facial muscles as a means of concealing wrinkles.

Botulin toxin has long been considered an ideal agent for chemical warfare (though, given its origins, the distiction from biological warfare is a thin one), since it oxidises rapidly on exposure to air, so an area attacked with a toxin aerosol would be safe to enter within a day or so. There are no documented cases of the toxin actually being used in warfare. There has been concern over the use of botulin toxin as a terrorist weapon, but it appears to not be ideal for this purpose. The vials of toxin used therapeutically are considered impractical for use by terrorists because each vial has only an extremely small fraction of the lethal dose for humans.

The toxin's properties did not escape the attention of the Aum Supreme Truth cult in Japan, who actually set up a plant for bulk production of this agent, though their terrorist and assassination attacks used the nerve agent Sarin instead, it being easier to disperse and faster acting.