The Douro (UK: /ˈdʊəroʊ, ˈdʊəruː/, US: /ˈdɔːruː, ˈdɔːroʊ/, Portuguese: [ˈdo(w)ɾu], Mirandese: [ˈdowɾʊ]; Spanish: Duero [ˈdweɾo]; Latin: Durius) is the largest river of the Iberian Peninsula by discharge. It rises near Duruelo de la Sierra in the Spanish province of Soria, meanders briefly south, then flows generally west through the northern part of the Meseta Central in Castile and León into northern Portugal. Its largest tributary (carrying more water than the Douro at their confluence) is the right-bank Esla. The Douro flows into the Atlantic Ocean at Porto, the second largest city of Portugal.

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.

The Latin name Durius might have been a Celtic name before the overwhelming Romanization of Iberia. If so, the Celtic root could have been *dubro- (in that family of languages the final vowel sound often changes with context, as in Latin). However, were that the case, the -b-, of which there remains no trace, would not have disappeared, as evidenced by place-names derived from Gaulish *dubron (plural dubra), such as French Douvres and English Dover (3rd/4th-century Dubris; Douvres in French), Spanish Dobra, German Tauber (Dubra-gave 807), and Gaelic/Old Irish dobur "water" and river name Dobhar in Ireland and Scotland. Modern Welsh dŵr "water" is cognate with modern Breton dour and Cornish dur "water" and results from a later typical Brittonic evolution of *dubro-, unknown in the Continental Celtic languages.

The possible origin is the hydronymic root *dur-, which is Pre-Indo-European or Pre-Celtic. Albert Dauzat linked this river name to a Pre-Celtic hydronymic root *dor-, which is well attested in Western Europe: in France Doire, Doron, Douron, etc. and in Italy Dora, etc. The meaning of this element is, however, unknown.

A folk-etymological derivation suggests that the name comes from the Portuguese or Spanish term for "golden".

In Roman times, the river was personified as the god Durius.

Part of the drainage basin might have been severely depopulated in the 8th century. According to Claudio Sánchez-Albornoz this was a deliberate act by Alfonso I of Asturias for the defence of his Kingdom, which led the area to be named Repoblación.

The Douro vinhateiro ("vine-land") of the Douro Valley in Portugal, long devoted to vineyards, has been designated by UNESCO as a World Heritage Site. The wine was taken downriver in flat-bottom boats called rabelos, to be stored in barrels in cellars in Vila Nova de Gaia, just across the river from Porto. Nowadays port wine is transported there in tanker trucks.