A peroxy acid (often spelled as one word, peroxyacid, and sometimes called peracid) is an acid which contains an acidic −OOH group. The two main classes are those derived from conventional mineral acids, especially sulfuric acid, and the peroxy derivatives of organic carboxylic acids. They are generally strong oxidizers.

Peroxymonosulfuric acid (Caro's acid) is probably the most important inorganic peracid, at least in terms of its production scale. It is used for the bleaching of pulp and for the detoxification of cyanide in the mining industry. It is produced by treating sulfuric acid with hydrogen peroxide. Peroxymonophosphoric acid (H₃PO₅) is prepared similarly.

Some peroxy acids are only hypothetical, but their anions are known. This is the case for peroxycarbonate and perborate (see sodium perborate).

Several organic peroxyacids are commercially useful. They can be prepared in several ways. Most commonly, peracids are generated by treating the corresponding carboxylic acid with hydrogen peroxide:

A related reaction involves treatment of the carboxylic anhydride:

This method is popular for converting cyclic anhydrides to the corresponding monoperoxyacids, for example monoperoxyphthalic acid.

The third method involves treatment of acyl chlorides:

meta-Chloroperoxybenzoic acid (mCPBA) is prepared in this way: