Lagoa dos Patos is a coastal lagoon in the state of Rio Grande do Sul, southern Brazil, and holds the distinction of being the largest lagoon in South America and the largest choked coastal lagoon in the world. It stretches approximately 290 kilometers from north to south, with a maximum width of 71 kilometers and a surface area of roughly 10,100 to 10,360 square kilometers. Its northern end connects to the Guaíba River near Porto Alegre, the state capital, while the southern end opens to the Atlantic Ocean through the Rio Grande channel at the city of Rio Grande.

The lagoon is relatively shallow, averaging around 5 to 6 meters in depth, which makes it susceptible to wind-driven waves and chop that can develop quickly across its vast open expanses. The sheer scale of the water body means that conditions can shift dramatically, and paddlers should treat open crossings with the same caution they would apply to coastal or open-water paddling. The southern portion of the lagoon transitions from freshwater to brackish conditions due to tidal exchange through the Rio Grande channel, while the northern reaches remain predominantly freshwater.

The lagoon is fed primarily by the Jacuí-Guaíba river system from the north and the Camaquã River, and it also receives overflow from Lagoa Mirim via the Canal de São Gonçalo. A sandbar roughly 8 kilometers wide, known as the São José do Norte Peninsula, separates the lagoon from the Atlantic Ocean, sheltering much of the lagoon from direct oceanic influence. Salt marshes, known locally as marismas, border the estuarine areas in the south and contribute significantly to the ecosystem's productivity.

The lagoon supports a notable artisanal fishery targeting species such as white croaker, mullet, and pink shrimp. Common bottlenose dolphins and other marine predators are occasionally observed hunting within the lagoon, and the area is recognized as an important habitat within the Pampa biome. Migratory birds are also associated with the wetland areas surrounding the lagoon.

Recreational vessels operating in Brazilian waters are required to be registered with the Brazilian Navy (DPC) and carry a valid Vessel Inscription Title (TIE/TIEM). The dredged commercial shipping channel connecting Rio Grande to Porto Alegre runs through the lagoon, so paddlers should be aware of commercial vessel traffic, particularly in the southern reaches near Rio Grande.