Knowledge of behaviours, including sex-biased dispersal and kin-association, provides important insight into the costs and benefits of group-living. Such behaviours can be difficult to observe directly, but have quantifiable genetic signatures that allow us to determine their occurrence within and among groups. The mourning cuttlefish Sepia plangon Gray, 1849 is both solitary and found in groups at different locations inside Sydney Harbour. Here we describe the genetic relatedness of individuals from four sampling locations, up to 7 km apart, within Sydney Harbour. We also describe the relatedness of juveniles swimming together in small groups of up to seven individuals and the relatedness of 32 adults found in 16 mating pairs, to evaluate the risk of inbreeding. We estimate relatedness with data from 52 amplified fragment length polymorphism (AFLP) loci. We found no evidence that groups of juveniles consisted of related individuals and therefore suggest that kin do not preferentially associate. Additionally, we found that relatedness between adult pairs did not differ from random. Taken together, these results suggest that the risk of inbreeding in this species is low. No genetic structure was detected among groups of juvenile S. plangon across our sampling locations, implying regular dispersal and gene flow among locations separated by distances of up to 7 km. We therefore suggest that group formation in S. plangon does not incur the risk of inbreeding or tend indirectly to benefit related individuals sharing the same group.