四氧化锇是化学式为OsO₄的化合物。尽管锇在地壳中含量稀少，四氧化锇仍有很多用途。它有很多有趣的性质，其中之一是挥发性。

纯四氧化锇是无色的，但因通常混有少量二氧化锇（OsO₂）而呈黄褐色。OsO₄可溶于四氯化碳（CCl₄），微溶于水，与水反应生成锇酸（见下文）。OsO₄具有挥发性，常温下易升华。气味与臭氧相似。锇的名称起源于希腊文的“osme”，意思是“臭味”。

Osmium tetroxide (also osmium(VIII) oxide) is the chemical compound with the formula OsO₄. The compound is noteworthy for its many uses, despite the rarity of osmium. It also has a number of interesting properties, one being that the solid is volatile. The compound is colourless, but most samples appear yellow. This is most likely due to the presence of the impurity OsO₂, which is yellow-brown in colour.