The Ebro (Spanish and Basque [ˈeβɾo] ^ⓘ; Catalan: Ebre, Western: [ˈeβɾe], Eastern: [ˈeβɾə]) is a river of the north and northeast of the Iberian Peninsula, in Spain. It rises in Cantabria and flows 930 kilometres (580 mi), almost entirely in an east-southeast direction. It flows into the Mediterranean Sea, forming a delta in the Terres de l'Ebre region, in southern Catalonia. In the Iberian peninsula, it ranks second in length after the Tagus and second in discharge volume, and drainage basin, after the Douro. It is the longest river entirely within Spain; the other two mentioned flow into Portugal.

The Ebro flows through many cities (Spanish: ciudades): Reinosa in Cantabria; Frías and Miranda de Ebro in Castile and León; Haro, Logroño, Calahorra, and Alfaro in La Rioja; Tudela in Navarre; Alagón, Utebo, and Zaragoza in Aragon; and Flix, Móra d'Ebre, Benifallet, Tivenys, Xerta, Aldover, Tortosa, and Amposta in the province of Tarragona (Catalonia).

The source of the river Ebro is in the Cantabrian Mountains, in Fontibre, Cantabria (Fontibre is from Latin Fontes Iberis, i.e. 'source of the Ebro'), just 42 kilometres (26 mi) from the Bay of Biscay, in the Atlantic Ocean. Close by is a large artificial lake, Embalse del Ebro, created by its damming.

The upper Ebro rushes through rocky gorges in Burgos Province. Flowing roughly eastwards it passes las conchas de Haro and begins forming a wider river valley among limestone rocks when it reaches La Rioja and Navarre. Southern tributaries rise on the nearby watershed, the Sistema Ibérico, a mountain range among those of Spain's centre. North of the headwaters is the Cantabrian mountain range (south of Bilbao and Santander). Much of the total water volume of the Ebro comes from its northern tributaries, which drain about 2⁄3 of the southern slopes of the Pyrenees mountains, which run along the border with France.

All of Andorra's drainage basin is in the Ebro, through the Segre.

Downstream from Cantabria the climate in the Ebro basin – the valley being isolated from sea air masses by surrounding mountains – becomes Mediterranean/Atlantic influenced and continental. This implies summer/winter seasonal contrast and a rather dry climate. Summers of the Ebro Valley most often mirror those of a semiarid climate; some of them see more breaks of storms or showers, yet others are more arid, bringing drought save where advanced irrigation is used.

The valley flood plains experience annual rainfall between 300 and 600 mm (12–24 in), with maxima in fall and spring. It is often covered with chaparral vegetation. Summers are hot and winters are cold. The dry summer season has temperatures of more than 35 °C (95 °F), occasionally reaching over 40 °C (104 °F). In winter, the temperatures often drop below 0 °C (32 °F). In some areas the vegetation depends heavily on moisture produced by condensation fog. It is a continental Mediterranean climate with extreme temperatures. There are many ground frosts on clear nights, and sporadic snowfalls.

The natural topsoils across the plains are, outside of historically well-forested zones, thin. The subsoil being close to the surface is organically poor and quick-draining: calcareous, pebbly, stony layers. These are in places salt-rich, seeing some saltwater endorheic lagoons.