Thapsigargin is the most widely used inhibitor of the ubiquitous sarco-endoplasmic reticulum Ca²⁺-ATPases in mammalian cells. A guaianolide compound of plant origin, it has become a commonly used research tool in the elucidation of the mechanisms of intracellular Ca²⁺ signalling.

Its remarkable potency and selectivity have been instrumental in developing our understanding of the function of intracellular Ca²⁺ stores, including store-operated Ca2+ entry or the involvement of the stores in protein synthesis or cell growth.