Szomolnokite (Fe<sup>2+</sup>SO<sub>4</sub>÷H<sub>2</sub>O) is a monoclinic iron sulfate mineral forming a complete solid solution with magnesium end-member kieserite (MgSO<sub>4</sub>÷H<sub>2</sub>O). In 1877 szomolnokite's name was derived by Joseph Krenner from its type locality of oxidized sulfide ore containing iron in Szomolnok, Slovakia (Hungary at the time).
As of mid-January 2020 the only continent on which szomolnokite has not been found and reported is Antarctica.
At room temperature szomolnokite is stable up to a pressure of 6.2 GPa, and then transforms into triclinic crystal structure.