Measurements of total iodine (I) and iodine-129 (¹²⁹I) concentrations in rivers and lakes of Argentina are presented. Their latitudinal distribution can be explained by taking into account their main sources (oceanic emissions and biomass burning for I, and atmospheric nuclear tests for ¹²⁹I), transport mechanisms, and fallout patterns. From the measured ¹²⁹I concentrations in the studied lakes, deposition fluences for their catchment areas were estimated. These results agree with a model of the global deposition pattern due to the ¹²⁹I released by atmospheric nuclear weapon tests and with other fluences reported for the southern hemisphere. In addition, the first measurements of ¹²⁹I in shallow seawater from the South Atlantic Ocean are presented and discussed.