The Uruguay River (in Spanish, Río Uruguay, ; in Portuguese, Rio Uruguai, [uɾuˈgwaj]) is a river in South America. It flows from north to south and makes boundary with Brazil, Argentina and Uruguay, separating some of the Argentine provinces of the Mesopotamia from the other two countries. It passes between the states of Santa Catarina and Rio Grande do Sul in Brazil; forms the eastern border of the provinces of Misiones, Corrientes and Entre Ríos in Argentina; and makes up the western borders of the departments of Artigas, Salto, Paysandú, Río Negro, Soriano and Colonia in Uruguay.
The river measures about 1,500 km in length and starts in the Serra do Mar (Brazil), at , where the Canoas River and the Pelotas River are joined, at about 200 m above mean sea level. In this stage the river goes through uneven, broken terrain, forming rapids and falls. Its course through Rio Grande do Sul is not navigable.
Together with the Paraná River, the Uruguay forms the Río de la Plata estuary. It is navigable from around Salto Chico. Its main tributary is the Río Negro, which is born in the south of Brazil and goes through Uruguay 500 km until its confluence with the Uruguay river, which is located 100 km north from the Uruguay's confluence with the Río de la Plata, in Punta Gorda (Colonia Department, Uruguay).
The river is crossed by four international bridges (from north to south): Paso de los Libres-Uruguaiana International Bridge, between Argentina and Brazil; and the Salto Grande Bridge, General Artigas Bridge and Libertador General San Martín Bridge between Argentina and Uruguay.
Argentina and Uruguay are currently experiencing a conflict over the Uruguay River. Two European companies proposed to build cellulose-processing plants at Fray Bentos, Uruguay, opposite Gualeguaychú, Argentina. Since April 2005, residents of the latter as well as many others have protested, claiming that the plants will pollute the river shared by the two countries. At the turn of the year the conflict escalated into a diplomatic crisis, making one of the companies move the project 250 km south. Since December 2005, the international bridges linking the Argentine province of Entre Ríos with Uruguay have been intermitently blockaded by Argentine protestors, causing major disruptions in commercial traffic and tourism.