Predictive models of species responses to climate change allow for the preventative management of many threatened taxa. All sea turtle species are currently threatened by climate change as they have temperature-dependent sex determination, meaning warmer nests will produce more female hatchlings and fewer males. In this study, we aimed to determine the trend in hatchling sex ratios for green (Chelonia mydas) and hawksbill turtles (Eretmochelys imbricata) at a nesting site in Papua New Guinea across a 140-year time series. Using environmental proxies to model past and future climates, we found that the overall trend in estimated sand temperature over the past 60 years suggests that the primary hatchling sex ratios have remained relatively balanced, unlike other equatorial nesting sites. Historically, the inter-seasonal variation in sand temperatures resulted in a notable increase in male hatchling production during cooler years. However, under a moderate climate change scenario, the model showed that there would be a ~1.5°C increase in sand temperatures for the region, resulting in a more female-biased hatchling sex ratio by 2100. Therefore, rookery managers in the future may need to prioritize the prevention of ‘feminisation’ by using shade, irrigation and positioning the hatchery to the coolest side of the island.