三氧化铀(UO₃)是铀的六价氧化物，可以通过在400℃灼烧硝酸铀酰得到。它的最常见的晶型是γ-UO₃，为橙黄色粉末。

三氧化铀可以在400℃和二氟二氯甲烷反应，产生氯气、光气、二氧化碳和四氟化铀；它和三氯氟甲烷反应生成的不是二氧化碳，而是四氯化碳。This is a case of a hard perhalogenated freon which is normally considered to be inert being converted chemically at a moderate temperature。

Uranium trioxide (UO₃), also called uranyl oxide, uranium(VI) oxide, and uranic oxide, is the hexavalent oxide of uranium. The solid may be obtained by heating uranyl nitrate to 400 °C. Its most commonly encountered polymorph, γ-UO₃, is a yellow-orange powder.