This article explores the Sun's influence on the hydrological cycle in southern South America (SSA) at the range of interannual-to-multidecadal scales from the early 1900s to 2011. The solar variability is described by the sunspot number (SSN) index. The hydrological cycle is examined by using annual mean discharges of the Paraná River (PAR) and the Atuel River (ATU) that represents the behaviour of the subtropical Argentinean Andean hydrological system. Wavelet-based methods are used in order to describe relationships in the entire time-frequency domain. The SSN–PAR connection is statistically weak in oscillations with period about 11 years (the Schwabe's solar cycle). Therefore, the solar forcing at this scale must be considered with great caution. The periodicity about 30 years is highly significant throughout the analysed period. Two potential physical mechanisms affecting the Paraná discharge could be involved: one is the solar irradiance influence on the Inter Tropical Convergence Zone (ITCZ), and the other is the solar influence on the Pacific long-term variability. The SSN–ATU connection shows by far the most striking, robust and convincing result for the Schwabe cycle. A large amplitude and statistically significant cycle with a period about 11 years is observed not only in the Morlet-based global and local wavelet spectra of the Atuel discharges and SSN but also in the global and local spectra of Cross and Coherent wavelets in most of the analysed period. High (low) discharges occur following maxima (minima) of the Schwabe cycles with time lags of up to ∼2 years. Previous studies have shown a close relationship between the subtropical Argentinean Andean Rivers and the El Niño/Southern Oscillation (ENSO), as well as a solar influence on the ENSO variability. We suggest that El Niño events occurring a few years after solar maxima could explain the connection. Periodicities longer than 30 years are suggested.