Soil erosion is intense in mountainous tropical regions where heavy storms result in the supply of large quantities of sediment to rivers. The origin and dynamics of suspended sediment were analysed in a catchment located in northern Laos during the first erosive flood of the rainy season in May 2012. The catchment was equipped with 4 successive gauging stations (draining areas ranging 0.2–11.6 km$^2$). Fallout radionuclides (Beryllium-7 – $^7$Be, unsupported Pb-210 –$^{210}$Pb$_{xs}$, and Cesium-137 – $^{137}$Cs) were determined on rainfall, overland flow, stream water, suspended sediment, soil surface and subsurface samples (with $n$ = 3, 19, 75, 75, 65 and 14 respectively). Assumptions underpinning the $^7$Be-labelling method were validated by implementing experiments in the laboratory (i.e., rainwater $^7$Be sorption to soil particles) and in the field (i.e., $^7$Be:$^{210}$Pb$_{xs}$ activity ratio evolution in rainwater and related overland flow during a natural storm event). Radionuclide analyses provided a way to quantify variations in sediment dynamics and origin throughout the flood: (1) a proportion of recently eroded sediment (labelled by $^7$Be, and referred to as “fresh sediment”) ranging between $ca$. 8–35% in suspended loads; (2) higher contributions of fresh sediment at the beginning of the flood rising stage; (3) a progressive dilution of fresh sediment by particles remobilised from the riverbed/channel; (4) the dominance of particles originating from the soil surface (ca. 70–80% of total sediment load) in upper parts and a much larger contribution of subsurface material ($ca$. 64%) at the downstream station. The original contribution of 7Be-labelled particles derived from collapsed riverbanks to sediment export was also demonstrated. This pilot study supports the use of fallout $^7$Be and $^{210}$Pb$_{xs}$ in tropical catchments to constrain sediment dynamics. It also puts forward the need to better characterize the sources of sediment in order to avoid possible misinterpretations.