三氧化硒（化学式SeO₃）是硒(VI)的氧化物。为无色潮解性晶体。

实验室通过用无水硒酸钾(K₂SeO₄)和三氧化硫反应制备三氧化硒： K₂SeO₄ + SO₃ → SeO₃ + K₂SO₄

三氧化硒在热力学上不稳定，逐渐转变为二氧化硒。

Selenium trioxide is the inorganic compound with the formula SeO₃. It is white, hygroscopic solid. It is also an oxidizing agent and a Lewis acid. It is of academic interest as a precursor to Se(VI) compounds.