Delphinids show a wide range of social structures; however few studies have investigated the potential influence of maternal kinship and genetic relatedness on school associations and sex composition. This study investigated kinship structure and sex composition 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 several schools of common dolphins. Each sample was used for sequencing a fragment of the mitochondrial DNA control region, genotyping at 14 nuclear microsatellite markers, and for genetic sexing. Pairwise analyses of individuals within and between schools, for both males and females, showed that individuals within schools were more likely to share mtDNA haplotypes. Average bi-parental genetic relatedness of female pairs and male pairs were also greater within schools than between schools. In addition, there was a trend for individuals of the same sex to be more likely found in the same schools, suggesting a degree of sex segregation in these dolphin schools. The results suggest that both maternal kinship and bi-parental relatedness have an impact on school associations of short-beaked common dolphins in southern Australia, similar to social traits reported to other dolphin species inhabiting shallow coastal environments. The information provided by this study contributes to our understanding of social evolution in delphinids, and reveals potential impacts dolphin-fishery interactions may have on the long-term viability and genetic diversity of common dolphins in southern Australia.