Catalase is a common enzyme found in nearly all living organisms exposed to oxygen. It catalyzes the decomposition of hydrogen peroxide to water and oxygen. It is a very important enzyme in reproductive reactions. Likewise, catalase has one of the highest turnover numbers of all enzymes; one catalase molecule can convert millions of molecules of hydrogen peroxide to water and oxygen each second.

Catalase is a tetramer of four polypeptide chains, each over 500 amino acids long. It contains four porphyrin heme (iron) groups that allow the enzyme to react with the hydrogen peroxide. The optimum pH for human catalase is approximately 7, and has a fairly broad maximum (the rate of reaction does not change appreciably at pHs between 6.8 and 7.5). The pH optimum for other catalases varies between 4 and 11 depending on the species. The optimum temperature also varies by species.

Catalase was first noticed in 1818 when Louis Jacques Thénard, who discovered H₂O₂ (hydrogen peroxide), suggested that peroxide's breakdown is caused by an unknown substance. In 1900, Oscar Loew was the first to give it the name catalase, and found its presence in many plants and animals. In 1937 catalase from beef liver was crystallised by James B. Sumner and the molecular weight was worked out in 1938.