In Temperate Mediterranean climates, the temperature increase due to global warming could improve the climatic suitability for subtropical fruit tree production. In Mediterranean environments, to prevent economic losses, it is essential to know the risk of frost damage before planting these species; however, most of the agroclimatic research is focused on the probability of occurrence of subzero temperatures. This information characterizes the general climatic variability, but not the potential damage for a specific crop. Therefore, this study aimed to formulate an algorithm of frost damage for avocado trees. For this purpose, the phenological cycle was divided into four phases having different sensitivity to frost: budbreak, vegetative growth in spring, summer-autumn foliage, and winter foliage. Sensitivity to frosts was represented by logistic curves, which were derived from threshold temperatures published by several authors. Weekly estimated damages were calculated through a multiplicative algorithm (Ihsem). By integrating weekly damage, a seasonal frost damage index (Ih) was calculated. To calibrate the seasonal frost damage, it was calculated over the whole Chilean Mediterranean climate area (32°02' S, 36°33' S). Temperature data was obtained from the agroclimatic grid modeled by the Center for Agriculture and Environment of the University of Chile (1 × 1 km) on the basis of ground stations. The results showed well-defined patterns of frost damage risk, progressively increasing from coastal areas to the Andes Mountains. The frost damage risk was higher on the southern coast and in the southern central valley. The values of Ih fluctuated between 0 and 0.08 in avocado production areas. Therefore, values of 0.03, 0.05 and 0.08 were considered low, intermediate, and high damage values, while values above 0.08 indicated areas with extreme damage risk where avocado trees cannot be grown. The longitudinal and latitudinal frost risk patterns were consistent with the climatic characteristics of the study area and the perception of local farmers, which confirmed the effectiveness of the index in representing frost damage of avocado plantations. Furthermore, Ihsem and Ih can be calibrated for any plant species, making it a valuable tool for assessing the impact of frost on crops and vegetation and facilitating bioclimatic zoning.