As a consequence of global warming, water temperatures in reservoirs are increasing and seasonal stratification patterns are changing. The question is to what extent the observed trends in epilimnetic water temperatures of reservoirs in Germany and Luxembourg are comparable and whether differences can be attributed to geographical and reservoir morphological factors. A multiple linear model with seasonal parameterization was fitted to water temperature data at 3m depth of 44 reservoirs over a period from 1993 to 2022. The model fits trend and seasonality well and can be used to estimate missing values. With the exception of a few dams with strong water transfer from other reservoirs, a linear increase in temperature was observed, with a median trend of 0.45 K per decade. The results reproduce the influence of altitude and latitude but showed also an effect of longitude as an indicator of continentality, with higher summer temperatures and lower winter temperatures in the east. Morphological factors also play a role. The higher heat storage capacity of large, deep reservoirs resulted in cooler epilimnion temperatures in spring and summer, and warmer temperatures in autumn. Overall, the results highlight the practical importance of climate-adapted reservoir management and regular monitoring.