Brazil–Uruguay border

From Cisplatine Province to Uruguay

By the Treaty of Incorporation of 31 July 1821, the Province of Montevideo, also called the Banda Oriental, was annexed by Brazil as its Cisplatine Province. After Brazilian forces were defeated at Ituzaingó in February 1827, Argentina and Brazil dropped their claims, and a treaty of 28 August 1828 created the Oriental Republic of Uruguay.^[1]

Brazil and Uruguay signed a boundary treaty on 12 October 1851, which cancelled the earlier treaties and set the line in more detail. Later treaties changed most of it: in 1852 between the Chuy stream and the Yaguarón river, in 1909 across Lagoa Mirim (Laguna Merín) and up the Yaguarón, and in 1913 on the San Miguel stream. Pillars marked the land stretches between 1920 and 1935.^[1]

“…descends by the said branch until it enters the Uruguay; the island or islands that are found in the embouchure of the said River Quareim into the Uruguay, belonging to Brazil.”^[1]

Which creek is the Invernada?

The 1851 treaty says the border follows the Cuchilla Negra ridge to the source of the Arroyo de la Invernada, then down that stream to the Cuareim. Uruguay says the Invernada is the more easterly of two streams and Brazil says it is the more westerly; the land between them, the Rincón de Artigas, is in dispute. A commission in 1852 couldn’t settle it.^[1]

In 1933 a Uruguayan officer on the joint commission, Captain Villa Seré, concluded that pillar 49-I had been placed at the head of the wrong stream, leaving about 25,000 hectares on the Brazilian side. Uruguay asked Brazil in 1934 to determine the stream scientifically, and Brazil refused. Since a 1974 decree, Uruguay’s official maps mark the area as a “contested boundary”.^[2]

Brazil’s last formal reply, in December 1989, kept the line as it is. In June 2025, when Brazil’s state company Eletrobras was building a wind farm in the area, Uruguay sent a note saying the project did not mean it recognised Brazilian sovereignty there.^[2]

Isla Brasilera

The 1851 treaty gives Brazil the island or islands at the mouth of the Cuareim. Uruguay argues that Isla Brasilera lies in the Uruguay river, not at the Cuareim’s mouth, and so is not covered. Because of the dispute, the tripoint with Argentina has no agreed location.^[1] Uruguay first protested formally in 1940, against a 1927 Brazil–Argentina convention that assigned the island to Brazil.^[1][2]

In 1979 the US Geographer knew of no negotiations on either dispute.^[1] El Observador described both claims in 2026 as “latent”, kept within diplomatic channels.^[2]