Sulfur trioxide (British spelling: Sulphur trioxide) has the chemical formula SO₃.

Sulfur trioxide is liquid, density 1.92 g/cm³, boiling point +43.7°C, melting point +16.8°C.

When mixed with water, it produces sulfuric acid (H₂SO₄):

SO₃ + H₂O = H₂SO₄     (+ 88 kJ)