Instead of the red blood of vertebrates, most molluscs have blue hemolymph containing hemocyanin, a type-3 copper-containing protein. The hemoglobin of vertebrate blood is replaced in most molluscs with hemocyanin, which plays the role of an oxygen transporter. Oxygen-binding in hemocyanin changes its hue from colorless deoxygenated hemocyanin into blue oxygenated hemocyanin. Molecules of molluscan hemocyanin are huge, cylindrical multimeric proteins—one of the largest protein molecules in the natural world. Their huge molecular weight (from 3.3 MDa to more than 10 MDa) are the defining characteristic of molluscan hemocyanin, a property that has complicated structural analysis of the molecules for a long time. Recently, the structural analysis of a cephalopod (squid) hemocyanin has succeeded using a hybrid method employing both X-ray crystallography and cryo-EM. In a biochemical breakthrough for molluscan hemocyanin, the first quaternary structure with atomic resolution is on the verge of solving the mystery of molluscan hemocyanin. Here we describe the latest information about the molecular structure, classification and evolution of the molecule, and the physiology of molluscan hemocyanin.