Social structure is a core element of population biology, influenced by intrinsic and environmental factors. Intra-taxon comparisons of social organization are useful in elucidating the role of such ecological determinants of sociality. Killer whales Orcinus orca are widely distributed, social delphinids with diverse morphology, diet, behaviour, and genetics, but few studies have quantitatively examined social structure in this species. We used 7 years of individual identification data on killer whales at Marion Island, Southern Ocean, to calculate the half-weight association index among 39 individuals, creating a weighted association network. There were long-term associations between individuals, though associations were dynamic over time. We defined 8 social modules using a community detection algorithm and these typically contained 3 individuals of various ages and sexes. Pairwise genetic relatedness among 20 individuals was not significantly correlated with association index. Individuals were on average more related within than between social modules, but social modules contained related as well as unrelated individuals. Likely parent pairs of 6 individuals indicated mating between social modules.