Climate change is anticipated to have major physical and biological effects on aquatic ecosystems. Changes to the thermal regime of lakes such as shorter duration of ice-cover, earlier onset of stratification and an increase in the length of the stratified period have already been observed in some lakes in North America and Europe. Using vertical temperature profiles of Lake Simcoe during the open-water period from 1980 to 2008 we evaluated trends in water column stability, the onset of stratification and the timing of fall turnover at three stations situated in the lake's two bays and its main basin. Dramatic changes were observed at the three stations; each stratified earlier in the spring, mixed later in the fall and remained stratified for more than a month longer in 2008 compared to 1980. Long-term monotonic trends in the timing of the onset of thermal stratification, fall mixing and duration of stratification were significant. These trends resulted from changes in water column density that are strongly correlated with increasing average air temperatures over time, indicating that climate warming has had a significant effect on the thermal regime of Lake Simcoe. © 2011.