Potassium sodium tartrate is a double salt first prepared (in about 1675) by an apothecary, Pierre Seignette, of La Rochelle, France. As a result the salt was known as Seignette's salt or Rochelle salt.

It is a colorless to blue-white salt crystallizing in the orthorhombic system. Its composition is KNa (C4H4O6)4H2O. It is slightly soluble in alcohol but more completely soluble in water. It has a specific gravity of about 1.79, a melting point of approximately 75° C, and has a saline, cooling taste.

It has been used medicinally as a purgative but in more recent years its piezoelectric properties have been more important and it has found usage in phonograph pickups and other sensing devices. It has also been used in the process of silvering mirrors. It is an ingredient of Fehling's solution, used in the determination of sugars in solutions.

Structural formula of Potassium sodium tartrate