The Douro is the largest river on the Iberian Peninsula by discharge. It originates near Duruelo de la Sierra in the Spanish province of Soria before meandering briefly south and flowing generally west through the northern part of the Meseta Central in Castile and León.
After entering northern Portugal, the river continues its westward course until it reaches the Atlantic Ocean at Porto, which is the second largest city in Portugal. Its largest tributary is the right-bank Esla.