Due to the many factors controlling δ13C values in stalagmites, complicating their paleoclimatic and paleoenvironmental interpretation, most studies do not present d13C values, but instead focus mainly on δ18O values. This is also the case for most cave studies from tropical South America, where many new δ18O stalagmite records covering the last millennia were recently published. Here, we review the d13C values in stalagmites, investigating the influence on this proxy of local hydroclimate, altitude, temperature and vegetation types, by employing a new dataset composed of published and unpublished carbon isotope records from various sites in tropical South America. The main factors influencing δ13C values are associated with the local hydroclimate, followed by minor effects from temperature. Most of the isotopic records show a significant correlation between the δ13C and δ18O values, indicating a close relationship between local hydroclimate and atmospheric convective processes related to the South American Monsoon System. The predominance of C3 plants above most of the karst systems studied here is responsible for the low δ13C values (≤6‰) in most of the speleothems, while local hydroclimate associated with prior calcite precipitation process is the main driver behind its variability during the last two millennia. Using Monte Carlo Principal Component Analysis, we produce an index of the mean hydrologic conditions and its changes over tropical South America for the last two millennia, which is closely related to monsoon variability for the period prior to 1750 CE. The recent break-down in the relationship between monsoon and local hydroclimate may have been caused by the increase in temperature, CO2, deforestation and fire during the current warm period.