Oldhamite is a calcium magnesium sulfide mineral with the chemical formula (Ca,Mg)S.[2][3] Ferrous iron may also be present in the mineral resulting in the chemical formula (Ca,Mg,Fe)S.[4] It is a pale to dark brown accessory mineral in meteorites.

It was first described in 1862 for an occurrence in the Bustee meteorite, Gorakhpur, Uttar Pradesh, India.

It was named for Irish geologist Thomas Oldham (1816–1878), the Director of the Indian Geological Survey.

[2][4] It occurs in association with enstatite, augite, niningerite, osbornite, troilite, gypsum and calcite.

It has also been reported from a slag occurrence in France and a coal deposit in Poland.