Fluvial export of particulate and dissolved 137Cs was investigated to reveal its sources and transfer mechanisms in a broadleaved forest catchment using a continuous collection system. The finest size fraction (<75 µm), consisting of decomposed litter and surface mineral soil, was the dominant fraction in the particulate 137Cs load, although the contribution of coarser size fractions increased during high water discharge in 2014. The dissolved 137Cs originated from the decomposition of 137Cs-contaminated litter. Temporal changes in 137Cs distribution in the litter–mineral soil system indicated that the dissolved 137Cs load will be moderated in several years, while particulate 137Cs load has the potential to continue for a long time.