The Nuclear Regulation Authority originally gave a level one rating - an "anomaly" - to the August 19 leak of 300 tons of water from a tank at the Fukushima Dai-ichi nuclear plant.
Last week it proposed raising that to level three - a "serious incident" - and has now made that change after consulting the International Atomic Energy Agency.
The ratings are designed to inform the international community and changing them does not affect efforts to clean up the leak by the government and plant operator Tokyo Electric Power Company (Tepco).
The 2011 Fukushima disaster itself was rated the maximum of seven on the IAEA's International Nuclear and Radiological Events Scale, the same as the 1986 Chernobyl accident.
Tepco has recovered some of the water that leaked from the tank but says some of it may have reached the sea through a rainwater gutter.