Physostigmine (also known as eserine from éséré, the West African name for the Calabar bean) is a highly toxic parasympathomimetic alkaloid, specifically, a reversible cholinesterase inhibitor. It occurs naturally in the Calabar bean and the fruit of the Manchineel tree.

The chemical was synthesized for the first time in 1935 by Percy Lavon Julian and Josef Pikl. It is available in the U.S. under the trade names Antilirium and Isopto Eserine, and as eserine salicylate and eserine sulfate. Today, physostigmine is most commonly used for its medicinal value. However, before its discovery by Sir Robert Christison in 1846, it was much more prevalent as an ordeal poison. The positive medical applications of the drug were first suggested in the gold medal-winning final thesis of Thomas Richard Fraser at the University of Edinburgh in 1862.