Identifying the observables that warn of volcanic eruptions is a major challenge in natural disasters management. An important, but under-investigated, observable is the diffuse heating of volcanic soils, which represents a major energy source at quiescent volcanism. However, it remains unclear whether diffuse heating, or surface warming, responds to pre-eruptive processes and varies before eruption. Here we show that the last eruptions/explosions of seven different volcanoes were preceded by a systematic long-term (~years) warming of their flanks. This warming, found through a new statistical analysis of satellite-based long-wavelength (10.780–11.280 μm) infrared data, is a phenomenon that operates over large areas (from a few to hundreds of km2) of the volcanic edifices. Specifically, the median temperature of the target volcanoes increased with respect to the surroundings by up to ~1.5 ℃ for several years before magmatic, phreatic, and hydrothermal eruptions. This reflects heat flux increases of up to ~10 W⁄m2 , probably driven by the enhancement of subsurface hydrothermal activity. Surface warming 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.