Phenotypic plasticity can allow individuals to compensate for potentially negative changes in their thermal environment. It is important, therefore, to understand the impacts of changes in both mean and fluctuations in temperature on plastic responses. Our aim was to establish the current state-of-knowledge regarding the influence of thermal variability on the capacity for phenotypic plasticity in ectothermic vertebrates and invertebrates. We conducted a quantitative synthesis of 44 studies (212 effect sizes across 40 species) to compare the effects of constant and fluctuating temperatures with the same mean on plasticity in biological responses, across different ecosystems (terrestrial and aquatic) and type of phenotypic plasticity (acclimation and developmental plasticity). We found that most studies implemented diel temperature fluctuations, and that phenotypic plasticity does not differ between constant and fluctuating thermal environments. We conclude that plasticity and its attendant compensation for thermal variability is driven by changes in longer-term mean temperatures.