The article discusses the current legal regime of South American rivers, the main stages of its formation and the principles that form its basis. There is a connection between the principle of freedom of navigation on international rivers, which has been enshrined in multilateral and bilateral international treaties between the states of South America and the process of regional integration in these countries. Particular attention is paid to the legal content of the principle that led to the creation of such an integration entity on the South American continent as MERCOSUR. It is noted that this principle was originally formulated with reference to European international rivers. Currently, the legal regime of the river systems of the Amazon and La Plata is regulated by a whole range of legal norms of both multilateral and bilateral international treaties, and the national law of coastal states. The article concludes that there is currently no single legal regime for navigation on international rivers on the South American continent. At the same time, this situation allows some lawyers to conclude that there is a common rule of international law on freedom of navigation for all states, at least in the Amazon and La Plata river systems.