The estuarine dolphin (Sotalia guianensis) is common along most of the Brazilian coast, but little is known about its reproduction. We analyzed the gonads of 50 individuals incidentally caught in fishing nets on the coast of Paraná, southern Brazil, between 1997 and 1999. Testes of adult males were ≤31.9 cm in length and ≤11.6 cm in width and ≤3.3% of the total body mass. Relative size of testes suggests a multimale mating system with sperm competition. Females ovulate in both ovaries, although the left ovary matures earlier than the right. Males reached sexual maturity at 7 years and at body lengths estimated at 170–175 cm. Females matured at 5–8 years of age and at body lengths of 164–169 cm. The reproductive cycle was estimated at 2 years, with no marked seasonality in ovulation or timing of birth. Gestation was about 12 months, fetal growth rate was 9 cm/month, and length at birth was estimated at 92.2 cm. Females older than 25 years had senescent ovaries.