A peroxy acid (often spelled as one word, peroxyacid, and sometimes called peracid) is an acid in which an acidic -OH group has been replaced by an -OOH group. They are formed chiefly by elements in groups 14, 15 and 16 of the periodic table, though boron and certain transition elements are also known to form peroxy acids. Sulfur and phosphorus form the largest range of peroxy acids, including some condensed form such as peroxydiphosphoric acid, H4P2O8 and peroxydisulfuric acid, H2S2O8.

Because they are generally very powerful oxidizing agents, most peroxyacids are prepared by electrolytic oxidation of ordinary oxyacids. Typically a high current density must be used to form the peroxyacid in good yield: the electrolysis of sulfuric acid being the most important example. Peroxy acids are generally not very stable in solution and decompose to ordinary oxyacids and oxygen, but relatively more stable anhydrous forms can be made by reacting hydrogen peroxide with chlorosulfonic acid or phosphoryl chloride.

All peroxy acids are very powerful oxidizing agents. Some such acids are peroxy-carboxylic acids, such as meta-chloroperoxybenzoic acid (mCPBA), which is used as a reagent in the Baeyer-Villiger oxidation and in oxidation of carbon-carbon double bonds in alkenes to generate epoxides (oxiranes). Sodium peroxy carbonate, Na2CO4, is widely used as a household bleaching agent for textiles, whilst sodium, potassium and ammonium peroxydisulfates (Na2S2O8, K2S2O8 and (NH4)2S2O8) are widely used as general purpose oxidizing agents in the laboratory.