Phthalic anhydride is the organic compound with the formula C6H4(CO)2O. It is the anhydride of phthalic acid.

Phthalic anhydride is a principal commercial form of phthalic acid. It was the first anhydride of a dicarboxylic acid to be used commercially. This white solid is an important industrial chemical, especially for the large-scale production of plasticizers for plastics.