Triuranium octoxide (U₃O₈) is a compound of uranium. It is present as an olive green to black, odorless solid. In spite of its color, it is one of the more popular forms of yellowcake and is shipped between mills and refineries in this form.

Triuranium octoxide occurs naturally as the olive-green-colored mineral pitchblende. U₃O₈ is readily produced from UF₆ and has potential long-term stability in a geologic environment. In the presence of oxygen (O₂), uranium dioxide (UO₂) is oxidized to U₃O₈, whereas uranium trioxide (UO₃) loses oxygen at temperatures above 500 °C and is reduced to U₃O₈. The compound can be produced by any one of three primary chemical conversion processes, involving either uranium tetrafluoride (UF₄) or uranyl fluoride (UO₂F₂) as intermediates. It is generally considered to be the more attractive form for disposal purposes because, under normal environmental conditions, U₃O₈ is one of the most kinetically and thermodynamically stable forms of uranium and also because it is the form of uranium found in nature. Its particle density is 8.3 g cm⁻³.