Knowledge of the mechanisms influencing phenology can provide insights into the adaptability of species to climate change. Here, we investigated the factors influencing multidecadal trends in the nesting phenology of the leatherback turtle Dermochelys coriacea at Playa Grande, Costa Rica, in the eastern Pacific Ocean and at Sandy Point, US Virgin Islands, in the western Atlantic Ocean. Between 1993 and 2013, the median nesting date (MND) at Playa Grande occurred later, at a rate of ~0.3 d yr . The opposing trends in the MND of each population were not explained by variation in the multivariate El Niño-Southern Oscillation index, North Atlantic Oscillation index, or Atlantic Multidecadal Oscillation index; however, the MND at Playa Grande was significantly correlated with nesting population size. We propose that changes in demography, linked to the population decline at Playa Grande, and the population recovery at Sandy Point may explain the contrasting trends in MNDs. If the observed trends in MND continue into the future, the nesting season at Playa Grande will coincide with increasingly adverse conditions for hatching success, exacerbating the already detrimental effects of climate change. Alternatively, shifts in the nesting phenology may make the Atlantic populations more resilient to climate change. Our findings highlight the increasing need for conservation efforts for eastern Pacific leatherback turtles to consider climate change mitigation practices.