Delphinids show a wide range of social structures. However, studies investigating the influence of genetic relatedness and maternal kinship on school associations are limited to a small number of relatively well-studied delphinid species. This study investigated biparental genetic relatedness and potential maternal kinship structure in schools of short-beaked common dolphins Delphinus delphis from southern Australian waters. A total of 128 biopsy samples were obtained from free-ranging individuals within 62 schools of common dolphins. Each sample was genotyped at 13 nuclear microsatellite markers, sequenced at 438 bp of the mitochondrial DNA control region, and used for genetic sexing. Pairwise analyses within and between schools showed that the average genetic relatedness between males was greater within schools than between schools. Pairs of individuals within schools were also more likely to share mtDNA haplotypes, for both pairs of males and of females. The results suggest that both biparental relatedness and potentially maternal kinship have an impact on school associations of short-beaked common dolphins in southern Australia, similar to social traits reported of other dolphin species inhabiting shallow coastal environments. The information provided by this study contributes to our understanding of social evolution in delphinids. It also suggests that dolphin bycatch and deaths in fisheries from this region could lead to a reduction in the genetic diversity of this population, particularly if related individuals are simultaneously killed in the nets.