This review is an exposition of published reports describing radioactive contamination of marine and freshwater fish, collectively revealing radiocesium contamination mechanisms. Radiocesium concentrations in marine fish caught in the waters off Fukushima have decreased drastically during the decade following the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident, mainly because of the area’s open and diffusive marine environments leading to lower radiocesium concentrations in seawater, sediments, and prey items, and because of the low physiological capability of retaining radiocesium in fish. Nevertheless, careful attention must be devoted to marine fish in the FDNPP port because some fish samples collected from the port have exceeded the Japanese regulatory limit of 100 Bq/kg-wet. Long-term monitoring of marine fish and surrounding environments is necessary because coastal waters around FDNPP have not reached an equilibrium state. Radiocesium contamination of freshwater fish has continued in rivers and lakes within areas with high degrees of deposition, although overall contamination levels have decreased considerably. Radiocesium contamination will be particularly persistent in lakes and ponds where dissolved radiocesium leaching from sediments can become a continuing source of contamination for biological magnification through the food web. In riverine environments, continuing supplies of contaminated prey items (e.g., insects) from riparian environments can be expected to engender persistent radiocesium contamination of freshwater fish (e.g., salmonid fish in forest rivers).