River water temperature is a key factor for water quality. Under climate change, inland water temperatures have increased, putting pressure on aquatic life and reducing the potential for human use. Here, future river water temperatures for Switzerland are studied. Results show that, towards the end of the 21st century, average river water temperatures will likely increase by 3.1 ± 0.7 °C. This is likely to increases the thermal stress on sensitive aquatic species such as the brown trout.