South America hosts one of the world’s largest hydrological systems, yet terrestrial water responses to warming and human influences remain poorly constrained. Here we compare observational and model-based datasets across 95 hydrological catchments, including the Amazon and La Plata basins, to assess trends and extremes in the South American water system during 1980-2010. All datasets consistently show no continental-scale intensification (no increase) of mean precipitation, runoff, or evapotranspiration, challenging expectations of generalized terrestrial water-cycle intensification under warming. Nevertheless, the datasets show shifts in water-flux seasonality and extremes between the first and second halves of the study period. Wet-season high flows increased in the Amazon Basin and dry-season low flows declined in the La Plata Basin, indicating increasing flood and drought risks, respectively. These results highlight a regionally differentiated, complex evolution of the South American hydroclimate.