Río de la Plata (Spanish for “Silver River”), also River Plate, estuary of the combined Paraná and Uruguay rivers in southeastern South America, forming a marine inlet between Uruguay and Argentina. The Paraná-Uruguay is the largest drainage system in South America after the Amazon. The Río de la Plata is 200 km (120 mi) wide at its mouth and tapers gradually inland for a distance of 300 km (190 mi) to the delta of the Paraná River.

The best natural harbor on the estuary is at Montevideo, Uruguay. Artificial harbors have been constructed for La Plata and Buenos Aires, Argentina, but their channels must be constantly dredged to prevent sand and silt from obstructing them.

The estuary was discovered in 1516 by Spanish explorer Juan Díaz de Solís and received its present name from Italian-born navigator Sebastian Cabot.