八氧化三铀（U₃O₈），是一种铀的化合物。它是一种橄榄绿到黑色的无臭固体。不管它的颜色如何，这是黄饼的其中一种更普遍的形态，和在磨坊与精炼厂之间运输的形态。自然界中，八氧化三铀(Triuranium octoxide)存在于橄榄绿色的沥青铀矿。UF₆和有潜力的长期稳定的地质环境中容易产生U₃O₈。有氧(O₂)存在时， 二氧化铀 (UO₂)氧化为U₃O₈，而三氧化铀(UO₃)在超过 500 °C 的高温时，会发布氧还原为U₃O₈。这个化合物可以用三种主要的化学转化过程中的任何一种制造，包括四氯化铀(UF₄)或氟化铀酰(UO₂F₂) 的中间体(intermediate)。以处置的目的而言，它通常被视为较具吸引力的形式，因为在一般的环境条件下，U₃O₈ 是一个最具动力学和热力学稳定形式的铀，也因为它是铀存在于自然界中的形式。它的粒子密度是 8.3 g cm.

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

U₃O₈ 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.