Thiomuscimol, also known as 5-aminomethyl-3-isothiazol, is a synthetic GABA<sub>A</sub> receptor agonist which is structurally related to the naturally occurring GABA<sub>A</sub> receptor agonist muscimol (found in Amanita muscaria). It is approximately equipotent with muscimol as a GABA<sub>A</sub> receptor agonist in vitro. Unlike muscimol however, thiomuscimol does not to any extent additionally act as a GABA reuptake inhibitor. On the other hand, like muscimol, it is a substrate for and metabolized by GABA transaminase (GABA-T), which is said to have limited its usefulness and application in animal behavioral studies. The drug was first described in the scientific literature by Povl Krogsgaard-Larsen and colleagues by 1976. It was encountered as a novel designer drug online in 2024.