Madagascar highlands, where the upper Ikopa River catchment is located, generally have rugged and complex landforms, making the soil vulnerable to erosion. Lacking cultivable surface, the farmers transform the hillslopes into terraces to protect and stabilize the soils. However, available data on this protection system efficiency are scare. In this work, fallout radionuclides (FRN), i.e., 137C and 210Pb, were used to estimate soil erosion rates at fields situated at natural slopes and terraces. These study sites were located using remote sensing and GIS tools. Soil cores up to 40 cm in depth were taken by motorized corer along transects from the top to the bottom of slopes following the runoff direction at the study sites and randomly at the reference site to investigate the soil redistributions due to the erosion process. In addition, a soil core sliced into 2 cm was collected at each site for the radionuclide depth distribution studies. Using conversion models, 137C and 210Pb specific activities, measured by gamma spectrometry, were converted into inventories to determine the soil erosion/deposition rates. At the slope fields, the 137Cs derived annual net soil loss varies from 7.4 to 9.1 t ha−1 a−1. The 210Pbex derived annual net soil loss varies from 5.9 to 6.9 t ha−1 a−1. At the terraced fields, the 137Cs derived annual net soil loss varies from 3.4 to 4.4 t ha−1 a−1. The 210Pbex derived annual net soil loss varies from 3.8  to 5.1 t ha−1 a−1. Compared to slope agriculture, these results demonstrate that terrace agriculture reduces the area impacted by erosion by 4–16%, the soil erosion rate by 52–54%, and the sediment delivery ratio by 25–31%. It strongly suggests that combining ancient and modern soil conservation approaches would be very useful for soil conservation, and traditional terracing could be combined with modern soil conserving land management measures (such as no tillage).