The Douro (UK: , US: , Portuguese: [ˈdoɾu]; Spanish: Duero [ˈdweɾo]; Latin: Durius) is the highest-flow river of the Iberian Peninsula. It rises near Duruelo de la Sierra in the Spanish province of Soria, meanders south briefly then flows generally west through the northern part of the Meseta Central in Castile and León into northern Portugal. Its most plentiful tributary (carrying a larger discharge than the Douro at their meeting) is the right-bank Esla river. At Douro's mouth at Porto, the second largest city of Portugal, the river meets the Atlantic Ocean.

The scenic Douro railway line runs close to the river. Adjacent areas produce port (a mildly fortified wine) and other agricultural produce. A small tributary of the river has the Côa Valley Paleolithic Art site which is considered important to the archaeological pre-historic patrimony, designated a UNESCO World Heritage Site. Within Spain, it flows through the middle of the autonomous community of Castile and León, with the basin spanning through the northern half of the Meseta Central. The latter includes wine producing areas such as the Ribera del Duero DOP.