It is observed that North American low-level jets (LLJs) over the eastern slopes of the Rockies occur mainly in boreal summer. In contrast, South American LLJs to the east of the Andes exist throughout the year and are strongest in austral winter. Understanding the causes of South American LLJs will help to reveal the processes that control the seasonal changes of the South American LLJs. In this study, we use the 15-yr (1979–93) European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis to investigate the mechanism responsible for South American LLJs. To represent the variability of the LLJs, an LLJ index is constructed by averaging the daily 850-hPa meridional winds over the LLJ region. The mechanism responsible for the South American LLJs is explored by examining the relationship between the LLJ index and large-scale circulation. It is demonstrated that the northerly LLJs to the east of the Andes are largely maintained by strong zonal pressure gradients locally, which are caused by a zonal flow crossing the Andes and lee cyclogenesis. The mechanism mainly operates in austral winter, spring, and fall. When the zonal flow is distorted by an anticyclonic circulation over the subtropical South Pacific, the northerly LLJs tend to reverse as observed. The dependence of the LLJs upon the upstream wind pattern provides an interpretation for the seasonal variation of the South American LLJs due to the seasonal changes of large-scale circulation patterns over the South Pacific. A method for up to 5-day forecasts of the LLJs based on 700-hPa zonal winds over the South Pacific was also introduced. A cross validation indicates a certain degree of predictability for South American LLJs. The results further suggest that in monitoring South American LLJs over the La Plata river basin, emphases should also be placed on the upstream flow pattern over the South Pacific.