Paraguay River, a river in central South America. From its source on the Mato Grosso Plateau in southwestern Brazil it flows generally southward through Paraguay to join the Paraná River just north of Corrientes, Argentina. Length is about 1,580 miles (2,540 km). Main tributaries are the Pilcomayo, Bermejo, and Sāo Lourenço. Asunción, capital of Paraguay, is the largest city on the river. The Paraguay River is navigable for small oceangoing ships as far as Asunción, but is used mostly by riverboats.