Polyhalite is an evaporite mineral, a hydrated sulfate of potassium, calcium and magnesium with formula: K₂Ca₂Mg(SO₄)₄·2H₂O. Polyhalite crystallizes in the triclinic system, although crystals are very rare. The normal habit is massive to fibrous. It is typically colorless, white to gray, although it may be brick red due to iron oxide inclusions. It has a Mohs hardness of 3.5 and a specific gravity of 2.8.

It occurs in sedimentary marine evaporites and is a major potassium ore mineral in the Carlsbad deposits of New Mexico. It is also present as a 2–3% contaminant of Himalayan salt.

Polyhalite was first described in 1818 for specimens from its type locality in Salzburg, Austria. The name comes from the German Polyhalit, which comes from the Ancient Greek words πολύς (polys) and ἅλς (hals), which mean "many" and "salt", and the German ending -it (which comes from the Latin ending -ites, which originally also came from Greek), which is used like the English ending -ite to form the names of certain chemical compounds.

Despite the similarity in names between polyhalite and halite (the naturally occurring form of table salt), their only connection is that both are evaporite minerals. The use of the Greek words for many and salt in polyhalite is due to polyhalite consisting of several metals that can form salts in the more general sense of the word salt used in chemistry.