Identifying the observables that warn of volcanic eruptions is a major challenge in natural hazard management. An important, but under-investigated, observable is the release of heat through volcano surfaces, which represents a major energy source at quiescent volcanism. However, it remains unclear whether surface heat emissions respond to pre-eruptive processes and vary before eruption. Here we show that the last magmatic and phreatic eruptions of five different volcanoes were preceded by subtle but significant long-term (~years), large-scale (10s of km2), increases of their radiant heat flux (up to ~1 of median radiant temperature). This pre-eruptive thermal unrest is found through a new statistical analysis of satellite-based long-wavelength (10.780–11.280 ) infrared data, and is attributed to the enhancement of underground hydrothermal activity. Large-scale thermal unrest is detected even before eruptions that were impossible to anticipate through other geophysical/geochemical methods (e.g., the 2014 phreatic eruption of Ontake, Japan; the 2015 magmatic eruption of Calbuco, Chile), thus opening new horizons to better constrain the thermal budget of volcanoes and improve eruption forecasts.