The effects of a 50% forest thinning intensity on Fukushima-derived 137Cs deposition by litterfall and its discharge by runoff in hillslope coniferous forest were monitored using four litterfall traps and a hillslope erosion plot. The observation was underway during the pre-and post-thinning periods. Results demonstrated that during the pre-thinning period a total 150 ± 13 g m−2 of litterfall deposited about 924 ± 69 Bq m−2 of 137Cs. This accounts for 11% of the local 137Cs fallout recorded for the study site in the aftermath of the accident. After thinning, both litterfall and 137Cs increased by more than six- and two-fold, respectively. This is possibly owing to the slow individual tree recovery rate assisted by the change on the running space provided by canopy openings, which can accelerate even the normal gust wind to gain damaging power on the unshielded mechanically injured parts of the contaminated residual trees. In both cases, litterfall generally transferred about 37% (3 ± 0.2 kBq m−2) of the local 137Cs fallout onto the forest floor over the observation period. The eroded litter-associated 137Cs increased by about a factor of two after thinning, which only accounted for less than 1% of 137Cs deposited by litterfall. This implies that the forest floor retains 137Cs and remains contaminated regardless of the size of the eroded litter material. But this could become a potential secondary contamination source for the downstream resources such as water bodies and villages, especially at the time of flooding, which in turn calls a serious attention in designing decontamination schemes.