Portugal has ten major rivers, five of which have their origins in Spain. The Rio Minho begins in Spanish Galicia and for a distance of seventy-four kilometers forms the northern Portuguese frontier with Spain. The Rio Douro is of great importance to the commerce of northern Portugal. It also originates in Spain and flows the entire width of Portugal before emptying into the Atlantic at Porto, the country's second largest city. The Rio Douro is navigable by small craft for its full distance of 198 kilometers in Portugal; historically the river was used to transport casks of port wine to Porto. Its steep banks are terraced with vineyards, and the valley of the Rio Douro is one of the most picturesque in all Portugal.

The Rio Tejo is the country's longest river, has the largest drainage basin, and is the most important economically. It is navigable only eighty kilometers upstream, but that includes the vast estuary on which Lisbon is located. The Tejo estuary is the best natural port on the European continent and able to handle large ocean-going vessels. It also contains the Cacilhas drydocks, the largest in the world.

The most important river in the south is the Rio Guadiana which, flowing north to south, forms part of the border with Spain. Other important rivers in Portugal include the Rio Lima and the Rio Tâmega in the north, the Rio Mondego in the center, and the Rio Sado and Rio Chança in the south.