A critical part of the sexual selection process in animals is the genetic mating system. Quantifying mating systems, especially in species with cryptic life-histories can be challenging. One approach is to use genotypic markers and accurate parentage analysis, along with methods to account for bias when sampling natural populations, to calculate sexual selection metrics derived from Bateman’s principles. In this study, three microsatellites were used to genotype 48 adults (23 female and 25 male) and 342 offspring from known mothers of live-bearing bluntnose klipfish. Parentage analysis was performed to interpret mating and reproductive success for both sexes. Metrics quantified were the opportunity for selection (I), the opportunity for sexual selection (Is), absolute (βss) and standardised (β’ss) Bateman gradients and the maximum intensity of precopulatory sexual selection (s’max). Multiple mating by both sexes were revealed by parentage analysis. However, females did not show significant Bateman gradients or a significant maximum intensity of precopulatory sexual selection (s’max), whereas male sexual selection metrics were all significantly greater than zero. These results suggests a polygynandrous mating system for this species. There is an opportunity for sexual selection to act on males but not females in this population, which is evolutionary tied to anisogamy, parental investment and sex-roles.