Nothofagus alessandrii is an endangered species that is naturally distributed in a Mediterranean environment in central Chile. In recent years, this territory has been subject to the effects of climate change, especially an increase in summer temperatures and prolonged periods of drought. In the summer of 2017, there was a fire of great magnitude consuming 184,000 ha, which affected the forests of N. alessandrii. This study assessed the severity and recovery dynamics of postfire vegetation by using spectral indices from Sentinel-2 images. The differenced normalized burn ratio (dNBR), relative differenced normalized burn ratio (RdNBR), and relativized burn ratio (RBR) were calculated before and after the fire and, later, the normalized difference vegetation index (NDVI) before the fire and during three consecutive years after the fire was utilized. The accuracy of the fire severity classifications was estimated using the kappa test (p<0.05). The three severity indices showed a similar classification in severity assessment and postfire response. The low-medium burn area in N. alessandrii forests ranged between 111.2 ha (RdNBR) and 130.3 ha (dNBR), and the high effect was between 46.1 ha (dNBR) and 66.0 ha (RdNBR), which was equivalent in both cases, approximately 11% of the total. Regarding the NDVI, vegetation recovery after three years of the fire showed a systematic return to prefire conditions. The assessment of the effect of a mega forest fire on the remaining forests of N. alessandrii based on Sentinel-2 images offers the opportunity for a better understanding of the severity of damage and the behavior of vegetation after the fire. All this information will help in a better recovery of these forests.