Cytochrome c oxidase (also known as Complex IV) is an essential enzyme in the mitochondrial respiratory chain, found in the mitochondria of eukaryotic cells. It is responsible for the final step in oxidative phosphorylation, in which electrons are transferred from cytochrome c to molecular oxygen. This process leads to the formation of water and the generation of a proton gradient across the inner mitochondrial membrane, which is subsequently utilised for the synthesis of adenosine triphosphate (ATP). ATP is the primary source of energy for most cellular processes.

Cytochrome c oxidase also plays a role in photobiomodulation (PBM), a therapy that uses light sources such as lasers or LEDs to influence biological processes and promote healing. The enzyme contains chromophores capable of absorbing light, which in turn influences mitochondrial activity and ATP production.