The threadfin rainbowfish (Iriatherina werneri Meinken 1974) is the sole species of the genus Iriatherina and was first discovered in Merauke Regency in 1973. This research aimed to examine the sex ratio, size at first gonad maturity, and spawning season of the threadfin rainbowfish (I. werneri) in Rawa Biru Swamp and Wanggo River, Merauke Regency, South Papua. The research was conducted for six months, from November 2022 to April 2023. In the meantime, the research employed exploratory research where the data analyzed were sex ratio, size at first gonad maturity, and gonad maturity stages. The findings denoted that overall the sex ratio of male and female threadfin rainbowfish found in Rawa Biru is 0.87:1.00 and in the Wanggo River is 0.96:1.00. The chi-square test results showed that the number of male and female fish in Rawa Biru is balanced, following a pattern of 1.00:1.00 (χ2value = 0.1437, χ2table = 9.4877).  The male threadfin rainbowfish in Rawa Biru mature their gonads at a total length of 26.23 mm, while female fish mature at 25.86 mm. In Wanggo River, male fish mature first at a size of 27.99mm and female fish at a size of 24.83mm. Based on the results of analysis of the GML threadfin rainbowfish in Rawa Biru and Wanggo River, generally the male and female fish caught are in the reproductive period. The sex ratio of the threadfin rainbowfish in Rawa Biru Swamp and Wanggo River was balanced, following a pattern of 1.00:1.00. Female fish in Rawa Biru Swamp and Wanggo River reached first gonad maturity faster than male fish.  Additionally, the spawning season of the threadfin rainbowfish occurs during the rainy season. The results indicate that the sex ratio of the threadfin rainbowfish in Rawa Biru and Sungai Wanggo is balanced, following a 1.00:1.00 pattern. Female fish in both Rawa Biru and Sungai Wanggo reach gonadal maturity more quickly than males. The spawning season for threadfin rainbowfish takes place during the rainy season.