•First study that uses Sentinel-2 data in all Galicia and Portugal wildfires (> 500 ha) occurred in October 2017.•EFFIS data overestimates burned areas in comparison to Sentinel-2 dNBR index.•EFFIS and Sentinel-2 burn severity levels results have a high correlation between them. A methodology to estimate the extent of areas affected by forest fires, as well as the burn severity levels using Sentinel 2 images (10 and 20 m) is proposed and applied to the fires occurred in October 2017 in Spain and Portugal. An extension larger than 250,000 ha and 4 burn severity levels (low, moderate, high and very high) have been obtained. The comparison with the European Forest Fire Information System (EFFIS), which uses MODIS images (250 m), shows that the methodology improves the area estimate by 10 % in commission area. In terms of burn severity levels, the Separability index (SI) and the Kappa statistic (k) show a high correlation between Sentinel-2 and EFFIS (SI values higher than one in all cases and k higher than 0.69, respectively).