Hemerythrin (also spelled haemerythrin; Ancient Greek: αἷμα, romanized: haîma, lit.

'red') is an oligomeric protein responsible for oxygen (O2) transport in the marine invertebrate phyla of sipunculids, priapulids, brachiopods, and in a single annelid worm genus, Magelona.

The names of the blood oxygen transporters hemoglobin, hemocyanin, and hemerythrin do not refer to the heme group (only found in globins).

The iron atoms are bound to the protein through the carboxylate side chains of a glutamate and aspartates as well as through five histidine residues.

The binding of O2 is roughly described in this diagram: Deoxyhemerythrin contains two high-spin ferrous ions bridged by hydroxyl group (A).

This proton-transfer result in the formation of a single oxygen atom (μ-oxo) bridge in oxy- and methemerythrin.

Then electrons are transferred from the ferrous ions to generate the binuclear ferric (Fe3+,Fe3+) centre with bound peroxide (C).