五氧化二氮（化学式：N₂O₅），又称硝酐，是硝酸的酸酐。通常状态下呈无色柱状结晶体，均微溶于水，水溶液呈酸性。可以用P₂O₅将浓HNO₃脱水得到。

五氧化二氮很容易潮解，而且在10℃以上能分解生成毒气二氧化氮及氧气，但在-10℃以下时较稳定。室温下易挥发、对光和热敏感。溶于热水时生成硝酸。遇高温及易燃物品，会引起燃烧爆炸。由五氧化二氮引起的火灾，可使用水或泡沫进行扑灭。

Dinitrogen pentoxide is the chemical compound with the formula N₂O₅. Also known as nitrogen pentoxide, N₂O₅ is one of the binary nitrogen oxides, a family of compounds that only contain nitrogen and oxygen. It is an unstable and potentially dangerous oxidizer that once was used as a reagent when dissolved in chloroform for nitrations but has largely been superseded by NO₂BF₄ (nitronium tetrafluoroborate).