Iron(II) sulfate (FeSO₄) is an example of an ionic compound. It is found in various states of hydration (FeSO₄·H₂O, FeSO₄·4H₂O, FeSO₄·5H₂O, FeSO₄·7H₂O); the heptahydrate is also called green vitriol, copperas, or melanterite (a mineral that commonly occurs with pyrite). Iron(II) sulfate has a blue-green color, monoclinic crystal structure, and is water-soluble. Its molecular weight is 151.9026 g/mol. Its melting point is 64°C, and at 90°C it loses water of hydration to form the monohydrate, a white powder known as the mineral szomolnokite when it occurs naturally. Iron sulfate pentahydrate forms the mineral siderotil.

Iron(II) sulfate is prepared commercially by oxidation of pyrite or by treating iron with sulfuric acid. It is used in the manufacture of inks, in wool dyeing as a mordant, and in water purification as a substitute for aluminium sulfate. It can also be used to treat iron deficiency.