The reproductive biology of the king weakfish, Macrodon ancylodon, inhabiting the estuarine waters of the Río de la Plata (Argentina–Uruguay), was studied using macroscopic and histological analysis of the gonads. The samples were collected from commercial landings and from three research cruises carried out between 1999 and 2001. King weakfish is a multiple spawner with indeterminate annual fecundity. Spawning of this species extends from October to March with two main peaks of gonadal index in November and March. Based on the incidence of females with new postovulatory follicles, the evening near sunset was the estimated time of spawning for M. ancylodon. Spawning frequency, determined using the percentages of females with postovulatory follicles, was about 17% during March 2001, indicating that females spawn once every six days at the second peak of the reproductive season. Batch fecundity ranges from 12,400 to 225,700 hydrated oocytes and was fitted to a linear function of total length and ovary-free female weight. The number of hydrated oocytes decreased at the end of the spawning season. In addition, a decline in hydrated oocyte size and weight was also observed during this period.