Saxitoxin (STX) is a cyanotoxin found in marine dinoflagellates (algae). It is a neurotoxin that is a selective sodium channel blocker. The United States military isolated saxitoxin and assigned it the chemical weapon designation TZ. It is unique among toxins in that it acts in a matter of minutes. The median lethal dose (LCt₅₀) of TZ is 5 mg·min/m³.

The medical importance of saxitoxin is in relation to red tide in shellfish and causes the paralytic shellfish poisoning (PSP) food poisoning. The blocking of the sodium channel produces a flaccid paralysis that leaves its victim calm and conscious through the progression of symptoms. Death is from respiratory failure.

It is listed in schedule 1 of the Chemical Weapons Convention. Though its early isolation and characterization were related to military efforts, saxitoxin has been more important to cellular research in delineating the function of the sodium channel.

In episode 22 of the television series “Prison Break”, a “glycoside saxitoxin hybrid” is used to cause cardiac arrest of a public official.