Magnesium sulfate is an inorganic salt with the formula MgSO4(H2O)x where 0 ≤ x ≤ 7. It is often encountered as the heptahydrate sulphate mineral epsomite (MgSO4·7H2O), commonly called Epsom salt.

A variety of hydrates are known.

The heptahydrate (epsomite) readily loses one equivalent of water to form the hexahydrate. Epsom salt takes its name from a bitter saline spring in Epsom in Surrey, England, where the salt was produced from the springs that arise where the porous chalk of the North Downs meets non-porous London clay.

The monohydrate, MgSO₄·H₂O is found as the mineral kieserite. It can be prepared by heating the hexahydrate to approximately 150 °C. Further heating to approximately 200 °C gives anhydrous magnesium sulfate. Upon further heating, the anhydrous salt decomposes into magnesium oxide (MgO) and sulfur trioxide (SO₃).

The heptahydrate can be prepared by neutralizing sulfuric acid with magnesium carbonate or oxide, but it is usually obtained directly from natural sources.