Phenological changes (i.e. shifts in the timing of biological events) are among the most frequently reported population-level responses to climate change and are often assumed to be adaptive and increase population viability. These may be driven by both individual-level phenotypic plasticity and population-level evolutionary and demographic changes. However, few studies have explored how individual-level versus population-level processes drive phenological trends. Using a 31-year dataset of over 600 individually marked nesting green turtles ( Chelonia mydas ), we quantify the population- and individual-level temporal trend in their first nest date. Of the latter, approximately 30% is attributable to individual phenological plasticity in response to sea surface temperature, with females advancing their nesting by 6.47 days for every degree (Celsius) increase. The remaining change is almost entirely explained by individual- and population-level changes in size and breeding experience (correlates of age), as well as the number of clutches laid per season. This is the first study of individual-level phenological change in a marine ectotherm, furthering our understanding of how this and similar species may respond to rising temperatures.