Jacobsite is a manganese iron oxide mineral. It is in the spinel group and forms a solid solution series with magnetite. The chemical formula is (Mn,Mg)Fe<sub>2</sub>O<sub>4</sub> or with oxidation states and substitutions: (Mn<sup>2+</sup>,Fe<sup>2+</sup>,Mg)(Fe<sup>3+</sup>,Mn<sup>3+</sup>)<sub>2</sub>O<sub>4</sub>.
It occurs as a primary phase or as alteration of other manganese minerals during metamorphism of manganese deposits. Typical associated minerals include hausmannite, galaxite, braunite, pyrolusite, coronadite, hematite and magnetite. It is a ferrimagnetic substance, which is weakly attracted by a magnet.
It was first described in 1869 and named for the Jakobsberg Mine, Nordmark, Filipstad, Värmland, Sweden.