Ullmannite is a nickel antimony sulfide mineral with formula: (NiSbS). Considerable substitution occurs with cobalt and iron in the nickel site along with bismuth and arsenic in the antimony site. A solid solution series exists with the high cobalt willyamite. It is steel-gray to tin white in colour with a metallic luster. It has a Mohs hardness of 5 to 5.5 and a specific gravity of 6.65. It crystallizes in the isometric crystal class and typically exhibits cubic, octahedral, or pyritohedral forms although euhedral crystals are rare. Crystal twinning is common.

Principal localities are in Germany, it is also found in Austria, Australia, France, England, and Wales.

It was discovered in 1843 at the Storch und Schöneberg Mine, North Rhine-Westphalia, Germany and named for the German chemist and mineralogist, Johann Christoph Ullmann (1771-1821).