A sample of unharvested rice contained 500 becquerels of cesium per kilogram, they said. Radioactive cesium was spewed from the Fukushima Dai-ichi nuclear power plant after it was damaged by a massive earthquake and tsunami on March 11.
Under Japanese regulations, rice with up to 500 becquerels of cesium per kilogram is considered safe for consumption.
Officials have tested rice from more than 400 spots in Fukushima prefecture. The highest level of cesium previously found was 136 becquerels per kilogram, prefectural official Kazuhiko Kanno said.
News of the elevated radiation level in rice from Nihonmatsu city, 55 kilometers (35 miles) west of the nuclear plant, set off alarm in the Japanese media.
The government has been testing vegetables and fish for radiation since the disaster, in which backup generators and cooling systems failed at the plant and the cores of three reactors melted.
Some nations have stopped importing some food products from Japan. Japanese consumers are nervous about radiation, but campaigns to buy from Fukushima have drawn support around the nation.