Studies have revealed an unsuspected complexity in social systems within a few lizard species, including group living, long-term monogamy and individual recognition of partners or offspring. Comparisons among these species and their relatives could provide valuable insights, allowing us to investigate traits that are shared across social systems and identify general principles relating to the evolution of sociality. The endangered pygmy bluetongue lizard, Tiliqua adelaidensis, is a member species in the Egernia group, but is thought to show a more solitary social structure than other members in this group. Within this study we used microsatellite markers to determine the mating system of T. adelaidensis. Unlike many other species in the Egernia group, we found a predominately promiscuous mating system in T. adelaidensis. We detected multiple paternity in 75 % of litters. Of the 70 males identified as having fathered juveniles, only 5 were identified as mating with the same female in more than 1 year and only 3 were identified as the father of juveniles with the same female in consecutive years. The genetic evidence suggested that partners were chosen randomly with respect to the level of relatedness among neighbouring lizards. However, mated lizards were geographically closer to each other than expected by random chance. Multiple paternities rely on the opportunity for males to encounter multiple females during the period when they are receptive to mating, and this may depend on population densities. Drivers for the polygamous mating system may be the single occupancy burrow and the central place territorial defence of those burrows in T. adelaidensis. We propose a fourth mating system for the Egernia group: polygyny within stable non-social colonies.