Nickel (III) oxide is the inorganic compound with the formula Ni<sub>2</sub>O<sub>3</sub>. It is not well characterized, and is sometimes referred to as black nickel oxide. Traces of Ni<sub>2</sub>O<sub>3</sub> on nickel surfaces have been mentioned.
Nickel (III) oxide has been studied theoretically since the early 1930s, supporting its unstable nature at standard temperatures. A nanostructured pure phase of the material was synthesized and stabilized for the first time in 2015 from the reaction of nickel(II) nitrate with sodium hypochlorite and characterized using powder X-ray diffraction and electron microscopy.