Mass movements and floods triggered by rainstorms are considered some of the most serious environmental problems and are quite often able to cause great disasters in large tropical cities. This has been occurring due to anthropic activities which change natural drainage and stability of slope in urban areas. However, lower monthly rainfalls which occur with high frequency can also cause mass movements and floods. The direct and indirect effects of less intense but continuous rainfall mainly affect less privileged communities, which occupy areas more sensitive to mass movements and flooding, such as locations along river margins, coastal lagoons, mangroves, and steep slopes. This manuscript highlights the important contribution of the lower monthly rainfall totals in triggering mass movements and floods on the east side of Pedra Branca Massif in the city of Rio de Janeiro. Geoprocessing was used to estimate the total monthly rainfall in rain gauges managed by the local authority; create surface interpolation and choropleth maps; and conduct a comparative analysis of histograms of the total monthly rainfall and flooding and mass movement records provided by the City's Civil Defence. The results confirmed the direct relation between higher monthly rainfall totals and mass movements and floods, and also showed that lower monthly rainfall totals can trigger a significant occurrence of such events. Moreover, the monthly rainfall limit that could trigger mass movements and floods was confirmed on the east side of Pedra Branca Massif.