In externally fertilized anurans, simultaneous polyandry occurs when sperm from multiple males compete to fertilize eggs from one female. Generally, females choose mates based on males’ characteristics (e.g. advertisement calls, color or body size), then form a mating pair (a primary male amplexus with a female) and finally search for an oviposition site. However, in species whose operational sex ratio is biased towards males, an un-paired male may adopt an alternative mating tactic, joining a mating pair as a peripheral male (polyandrous mating) and releasing sperm to obtain reproductive success by sperm competition. Simultaneous polyandry has been observed in the foam-nesting frog Rhacophorus prasinatus in Taiwan. In this study, we observed the mating behavior, collected clutches and conducted parentage analysis in R. prasinatus. Our main results were (1) that polyandrous mating led to clutches with multiple paternity, (2) that the primary male sired more offspring than did peripheral males and (3) that the frequency of clutches with multiple paternity in the wild was 86%. These results indicate that their alternative mating tactic indeed help peripheral males to obtain reproductive success, although their paternity share is less than that of primary males, and suggests that this tactic may present a solution to intense pressure from intra-sexual selection.