Depth-resolved water temperature data on the thermal environment of lakes are often hindered by sparse temporal frequency, limited depth resolution, or short duration that create many challenges for long-term analysis. Where high frequency and depth-resolved data exist, they can provide a wealth of knowledge about how lakes are responding to a changing climate. In this study, we analyzed 302 profiles of summer mean water temperature (July to September), which includes 6756 unique observations, from Lake Konnevesi (Finland) and investigated changes in lake surface and deep-water temperature from 1984 to 2021. Statistical analysis of this dataset suggests a substantial warming of lake surface water temperature (0.41 °C decade-1) but no significant change in the deepest layer (0.00 °C decade-1). Our analysis also suggested the inter-annual variability in lake surface temperature correlated significantly with the upward trends of summer mean air temperature and solar radiation, but suggested no significant effect of observed changes in ice break-up dates or changes in near-surface wind speed. None of the investigated predictors correlated with the change in deep-water temperature. Due to the variable response of lake surface and bottom water temperature to climate change in this high-latitude lake, our data suggest a substantial increase in lake thermal stability. Our study supports the growing literature on lake thermal responses to climate change, and illustrates the unique contrast of climate change impacts at the surface and at depth in lake ecosystems, with deep waters acting as a potential thermal refuge to aquatic organisms within a warming world.