Gonatus onyx is one of the most abundant cephalopods in the Pacific and Atlantic Oceans1 and is an important prey species for a variety of vertebrate predators2,3, but a full understanding of its life history has been hampered because spawning occurs at great depths4,5, where observation is difficult. Here we describe post-spawning egg care, or brooding, in this deep-sea squid. Our finding is unexpected because this behaviour differs from the reproductive habits of all other known squid species.