Reproductive phenology of snakes varies widely among species, likely influenced by biotic and abiotic factors related to seasonality. In Neotropical regions, where seasonal variation in temperature and photoperiod is minimal, the factors regulating the reproductive phenology of snakes remain poorly known, despite the great diversity of species found in this region. We hypothesise that the phenology of tropical aquatic snakes is directly influenced by local precipitation patterns and resulting water levels. We studied for the first time the reproductive cycle of a neotropical aquatic oviparous snake, Helicops pastazae, in a mountain river in the Andean foothills of Colombia. To do so, we sampled 212 snakes from 2013 to 2017 and classified the monthly reproductive stages of the adult snakes. In females we observed follicular development and presence of oviductal eggs, in males we performed histological analyses of testes, kidney, and deferent ducts. Females showed a marked seasonal reproductive cycle, with secondary vitellogenic individuals mainly clustered during the high precipitation months, ovigerous stages found throughout low precipitation months, and with egg-laying occurring from low precipitation to the early dry seasons. Males produced spermatozoa throughout the year indicating year-round reproduction at a population level. We found sexual dimorphism in tail length, with both juvenile and adult males showing significantly longer tails than females, while adult females were larger than males and showed greater midbody width, head length, and head width. The reproductive activity of H. pastazae is highly seasonal in females, driven by regional rainfall patterns, whereas males are reproductive thorough the year, being a permanent resource for females.