The largest lake in South America is Lake Maracaibo, lc.5,100 sq mi (13,210 sq km), NW of Venezuela, extending c.110 mi (180 km) inland 2 lake Titicaca, 3 lake PooPo. Lake Titicaca is the largest freshwater lake in South America.
Lake Tanganyika (is in Africa) The largest lake in south America is Lake Maracaibo followed by Lake Titicaca. Some sources call Titicaca the largest lake and treat Maracaibo as a sea inlet because it is coastal and salty in places.
You are thinking of Lake Maracaibo, which technically isn't a lake, because it is connected to the Gulf of Venezuela, (which is in the Caribbean Sea) by the Tablazo Strait. The largest proper lake in South America is Lake Titicaca on the border of Peru and Bolivia.