The Douro (UK: /ˈdʊəroʊ, ˈdʊəruː/, US: /ˈdɔːruː, ˈdɔːroʊ/,[1][2][3][4] Portuguese: [ˈdo(w)ɾu], Mirandese: [ˈdowɾʊ]; Spanish: Duero [ˈdweɾo]; Latin: Durius) is the largest river of the Iberian Peninsula by discharge.

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.

[6] 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.

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.

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.

In 1998, Portugal and Spain signed the Albufeira Convention, an agreement on the sharing of trans-boundary rivers to include the Douro, Tagus and Guadiana.

Except for Porto and Vila Nova de Gaia at the river mouth, the only population centres of any note are Foz do Tua, Pinhão and Peso da Régua.

Tributaries here are small, merging into the Douro along the canyons; the most important are Côa, Tua, Sabor, Corgo, Tavora, Paiva, Tâmega, and Sousa.

The most populous cities along the Douro River are Valladolid and Zamora in Spain, and Porto and Vila Nova de Gaia in Portugal.

The region around Pinhão and São João da Pesqueira is considered to be the centre of port, with its quintas (or farms/estates) that extend along the steep slopes of the river valleys.

Recently, a prosperous tourist industry has developed based on river excursions from Porto to points along the Upper Douro valley.

Fifteen dams have been built on the Douro to regulate the water flow, generate hydroelectric power, and allow navigation through locks.