The interaction between UV-B and temperature can modify the effects of climate variability on animal function because UV-B and increasing temperatures may increase reactive oxygen species (ROS) production and thereby impair animal performance. However, antioxidant enzyme activities are also increased at higher temperatures, which could counteract negative effects of increased ROS. Conversely, UV-B exposure at lower temperature can exacerbate the effects of ROS because of lower antioxidant enzyme activities. Phenotypes can be plastic to compensate for potentially negative environmental effects. Plasticity may be induced by conditions experienced during pre- or early post-zygotic development, and it may occur reversibly within adult organisms (acclimation). Developmental plasticity and acclimation may interact to determine phenotypes in variable environments. Here, we tested the hypothesis that increased antioxidant enzyme activities are insufficient to alleviate the interactive effects of UV-B and increased temperature on mosquitofish (Gambusia holbrooki). Additionally, we tested whether developmental conditions influenced the capacity for acclimation to UV-B and temperature so that cohorts born in summer at high UV-B and temperature conditions are better able to compensate for ROS damage compared to cohorts born in winter. We exposed mosquitofish to UV-B and control (no-UV-B) at different acclimation temperatures (18, 28 and 32 °C), and measured responses acutely at 18, 28 and 32 °C in a fully factorial design. In fish born in summer, UV-B had significant negative effects on swimming performance and resting metabolic rate at both low (18 °C) and high (32 °C) acclimation temperatures, which were accompanied by higher ROS-induced damage. At their average temperature experienced naturally (28 °C), fish born in summer were not affected by UV-B and showed lower damage and higher antioxidant enzyme activities compared to the other acclimation temperatures. In contrast, swimming performance of winter-caught fish was negatively affected by UV-B at all acclimation temperatures, which was paralleled by higher ROS-induced damage and antioxidant enzyme activities that did not acclimate. However, metabolic scope was not reduced by UV-B or temperature in any of the cohorts. Our results showed that developmental conditions modify the capacity for acclimation later in life, and that the interaction between developmental and acclimation conditions can increase the resilience of animals to environmental variability. These results have important implications for understanding the evolution of acclimation, and for predictions of how climate change affects animal performance.