The Río de la Plata, also called the River Plate or La Plata River in English, is the estuary formed by the confluence of the Uruguay River and the Paraná River at Punta Gorda. It empties into the Atlantic Ocean and forms a funnel-shaped indentation on the southeastern coastline of South America. Depending on the geographer, the Río de la Plata may be considered a river, an estuary, a gulf, or a marginal sea. If considered a river, it is the widest in the world, with a maximum width of 220 kilometres (140 mi).

An estuary is a partially enclosed coastal body of brackish water with one or more rivers or streams flowing into it, and with a free connection to the open sea. Estuaries form a transition zone between river environments and maritime environments and are an example of an ecotone. Estuaries are subject both to marine influences such as tides, waves, and the influx of saline water, and to fluvial influences such as flows of freshwater and sediment. The mixing of seawater and freshwater provides high levels of nutrients both in the water column and in sediment, making estuaries among the most productive natural habitats in the world.