The population structure and reproductive biology of the surubim of Iguaçu, Steindachneridion melanodermatum, a large endemic and endangered catfish from Iguaçu River basin, were evaluated. In total, 182 specimens of this species were sampled from September 2010 to December 2016 in the Lower Iguaçu River, including some tributaries. The determination of the cycle and reproductive period was undertaken by assessing the mean variation in the values of the gonadosomatic index (GSI) of females and males, as well as by the frequency distribution of the stages of gonadal development on the basis of macroscopic and microscopic analyses. The total length and bodyweight ranged from 21 to 102 cm and from 82.8 to 15 670 g respectively. The reproductive period of this species was from June to September (winter and early spring), and may extend until November, with GSI peaking in August for males and September for females. The size at first maturation (L50) corresponded to 39.5 cm for females and 43.9 cm for males. Spawning in this species was associated with lower water temperatures, and high discharge and dissolved oxygen concentration. Our findings are crucial in assisting the development of effective measures and specific criteria for the conservation of this endangered species.