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

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 but before its discovery by western medicine in 1846 it was much more prevalent as a poison. Physostigmine has an LD50 of 3 mg/kg in mice.