The threadfin rainbowfish (Iriatherina werneri Meinken, 1974) is a single species of the genus Iriatherina and was first discovered in Merauke Regency in 1973. This study aimed to examine the size distribution based on total length and body weight, length-weight relationship, and condition factors of the threadfin rainbowfish (I. werneri) in Rawa Biru and Wanggo River, Merauke Regency, South Papua. The study was conducted within six months, from November 2022 to April 2023. This exploratory research analyzed the distribution of total length and body weight, length-weight relationships, and fish condition factors.  The results indicated that male fish from Rawa Biru typically ranged from 27.87–30.95mm in total length, while females measured 24.79–27.86mm. In contrast, specimens from the Wanggo River were smaller, with males and females ranging from 26.17–28.14mm. Based on the length-weight relationship analysis, hypoallometric, isometric, and hyperallometric growth patterns were obtained in male and female fish in the Rawa Biru and female fish in the Wanggo River. In contrast, hypoallometric and isometric patterns were found only in male fish in the Rawa Biru. The value of male and female fish condition factors varies in feeding times.