This study presents an approach for estimating the offshore wind potential of Japan. Bathymetry data (1 km mesh) and near shore wind speed data of the year 2018 were used to assess the potential. A turbine with a peak power of 10.6 MW was employed for the analysis. The potential was calculated for multiple regions. These regions are based on the service areas of the major electricity supply companies in Japan. Overall, the results show that Japan has the potential to produce up to 32,028 PJ electricity per year. The electricity demand of 2018 amounts to 3231 PJ. The potential is therefore large enough to cover Japan's electricity needs ten-times over. The capacity that could theoretically be installed amounts to 2720 GW, which is a multiple of the current worldwide installed capacity of 29.1 GW (2019). In addition to the huge potential, the regional assessment shows that the regions vary greatly in their potential; of all the considered regions, Hokkaido and Kyushu have the highest overall potential.