Forest fires have intensified in the Mediterranean area over the last decades, becoming increasingly destructive. Catastrophic wildfires, such as the ones that occurred on the Portuguese territory during 2017, have emphasized the need for developing management tools capable of rapidly assessing their impact on downstream water bodies. This study focuses on the effects of the 2017 forest fires on the water quality of the Zêzere River (Portugal), one of the major sources of freshwater to Lisbon and the surrounding areas. During 2017, more than 100,000 hectares (30% of the Zêzere watershed) were affected by wildfires, making it one of the largest burned areas in Portugal. Using the Soil Water Assessment Tool (SWAT), the land use (curve number, crop vegetation management factor) and soil (soil erodibility factor) parameters were modified considering different magnitudes of the fire event (low, medium, and high severity). The impact of the different wildfires was then assessed at the sub‐basin level and at the entrance of Castelo de Bode reservoir. Simulations showed a significant increase in runoff and sediment concentration at the sub‐basin level in the years following the fire events, with high concentrations of nutrients occurring at the reservoir inlet. Nitrate concentration slightly overcame the threshold limits foreseen in legislation for drinking water, meaning that fires could have additional impact on human health. Phosphate concentration recurrently exceeded the legislation threshold, representing a severe risk to the ecosystem by potentially promoting the eutrophication of downstream water bodies, oxygen deficiency in the bottom substrate, and reduced biodiversity.