Well poisoning is the act of malicious manipulation of potable water resources in order to cause illness or death, or to deny an opponent access to fresh water resources.

Well poisoning has been historically documented as a strategy during wartime since antiquity, and was used both offensively (as a terror tactic to disrupt and depopulate a target area) and defensively (as a scorched earth tactic to deny an invading army sources of clean water). Rotting corpses (both animal and human) thrown down wells were the most common implementation; in one of the earliest examples of biological warfare, corpses known to have died from common transmissible diseases of the Pre-Modern era such as bubonic plague or tuberculosis were especially favored for well-poisoning.

Additionally, well poisoning was one of the three gravest antisemitic accusations made against Jews during the pre-modern period (the other two being host desecration and blood libel). Similar accusations were also made of Koreans living in Japan in the aftermath of the 1923 Great Kantō earthquake. In both cases the accusation was never substantiated, but did lead to wide-scale persecution and pogroms against the group so accused.