Lagoa Mirim, Coastal lagoon in Rio Grande do Sul, Brazil.

Lagoa Mirim is a coastal lagoon that straddles the border between Brazil and Uruguay in southern Rio Grande do Sul. The body of water connects to neighboring Patos Lagoon through the São Gonçalo Canal, allowing vessels to travel between these two water systems.

Until the early 20th century the lagoon belonged entirely to Brazil, leaving Uruguay without a share of its shore. After the diplomatic intervention of the Baron of Rio Branco, the 1909 Condominium Treaty ceded half of the lagoon and the Yaguarón River to Uruguay. On 31 August 1915 Uruguay renamed the town of Villa Artigas as Río Branco in his honour, and on 7 July 1977 the Basin Treaty was signed in Brasília, creating the joint commission (CLM) with offices in Treinta y Tres and Porto Alegre.

The name comes from Tupi-Guarani mi'rí, "small", also rendered in Nheengatu as Ipawa mirim, "little lake". On the Uruguayan shore, the balneario Lago Merín, 110 km from Melo, draws kitesurfers when spring winds last into early winter, and has a casino and viewpoints. Cattle raising and rice growing dominate both banks.

Visiting the lagoon works best in warmer months when weather favors water activities and exploration. Most access points are reached through small harbors or landings located in nearby settlements around the shoreline.

The lagoon's name comes from Tupi-Guarani "mi'rí", meaning "small", though it is South America's second-largest freshwater lagoon at 3,750 km², after Lake Titicaca. In Nheengatu it is called Ipawa mirim, "little lake".