Peroxidases or peroxide reductases (EC number 1.11.1.x) are a large group of enzymes which play a role in various biological processes. They are named after the fact that they commonly break up peroxides, and should not be confused with other enzymes that produce peroxide, which are often oxidases.

Peroxidases typically catalyze a reaction of the form:

$${\displaystyle {\ce {ROOR'+{\overset {electron \atop donor}{2e^{-}}}+2H+->[{\ce {Peroxidase}}]{ROH}+R'OH}}}$$

For many of these enzymes the optimal substrate is hydrogen peroxide, but others are more active with organic hydroperoxides such as lipid peroxides. Peroxidases can contain a heme cofactor in their active sites, or alternately redox-active cysteine or selenocysteine residues.

The nature of the electron donor is very dependent on the structure of the enzyme.

Protein families that serve as peroxidases include:

The glutathione peroxidase family consists of 8 known human isoforms. Glutathione peroxidases use glutathione as an electron donor and are active with both hydrogen peroxide and organic hydroperoxide substrates. Gpx1, Gpx2, Gpx3, and Gpx4 have been shown to be selenium-containing enzymes, whereas Gpx6 is a selenoprotein in humans with cysteine-containing homologues in rodents.

Amyloid beta, when bound to heme, has been shown to have peroxidase activity.