•Some buckwheat samples had relatively high radioactive Cs in eastern Japan in 2012.•K application effectively decreased the radioactive Cs in buckwheat.•Radioactive Cs in buckwheat decreased after the recommendation of K application in 2013. Tokyo Electric Power Company's Fukushima Dai-ichi nuclear power plant released radioactive cesium (Cs) into the environment after the Great East Japan Earthquake of March 11, 2011. After radioactive Cs fell onto agricultural fields, radioactive nuclide levels in some buckwheat (Fagopyrum esculentum Moench) grain subsequently exceeded new standard limits for radioactive materials, including that for radioactive Cs (100Bqkg−1) established by the Japanese government in 2012. A survey of soils and buckwheat grain from 68 farmers’ fields in 2012 revealed that soil exchangeable potassium (K) concentration was significantly and negatively correlated with radioactive Cs concentration in the grain. The effect of K application on the reduction of radioactive Cs concentration in the grain was confirmed by pot and field experiments conducted respectively in 2012 and 2013. This effect might result from the similarity of ion forms of K and Cs and/or the lower exchangeable Cs concentrations at higher exchangeable K concentrations in soil. Based on these results, farmers were recommended to establish a soil exchangeable K concentration of 250mgkg−1 (300mgkg−1 in K2O) before applying basal fertilizer. After this recommendation, the soil K concentrations of farmers’ fields increased, causing an overall decrease in radioactive Cs concentrations of buckwheat grain produced in 2013. Consequently, no grain sample from this year exceeded the standard limits for radioactive Cs level across the affected area in Japan. These efforts contributed greatly to the rehabilitation and reconstruction of agriculture in the area contaminated with radioactive Cs.