Thénardite is an anhydrous sodium sulfate mineral, Na2SO4 which occurs in arid evaporite environments, specifically lakes and playas.

It also occurs in dry caves and old mine workings as an efflorescence and as a crusty sublimate deposit around fumaroles.

It was first described in 1825 for an occurrence in the Espartinas Saltworks, Ciempozuelos, Madrid, Spain and was named for the French chemist, Louis Jacques Thénard (1777–1826).

Thénardite is fluorescent, white in shortwave and yellow-green in longwave UV radiation.

In humid conditions, thénardite progressively absorbs water and converts to the deca-hydrated mineral mirabilite, Na2SO4 · 10 H2O.