Medaka (Oryzias latipes) is a popular small aquarium pet fish in Japan, but it is also an experimental animal established and developed in Japan. Along with zebrafish, it has become an important model animal and is a powerful model vertebrate organism for developmental and molecular studies. Medaka has also been used to study the effects of radiation, and much work has been done using it. The radiosensitivity of medaka is approximately one-fifth that of mammals. The release of radionuclides from the Fukushima Daiichi Nuclear Power Plant (FNPP) accident in 2011 contaminated its surrounding area, and this contamination remains today in 2024. This review focuses on wild medaka living around FNPP and examines the following: (1) ambient dose rate of the habitat, (2) radioactivity concentration of the environmental media (medaka and sediment), (3) estimation of the absorbed dose from the environmental media using the Environmental Risk from Ionising Contaminants: Assessment and Management (ERICA) tool, (4) estimation of the exposure dose using dosimeters placed in the habitats, and (5) radiation effects using the micronucleus assay. Considering the radiosensitivity of medaka, estimated and measured absorbed doses, and actual effects, at present, no detectable radiation effects have been observed in wild medaka around FNPP.