The climatology and long-term trend of wave energy resources around Japan were estimated from in situ wave data measured at 25 observation points during 1980 to 2009. The climatological annual mean wave energy around Japan was estimated to be 6.4 kW m−1, and the long-term trend was 0.27 kW m−1/30-yr. Our results indicate that the long-term trend in wave energy varies among areas and seasons. There is a marked increasing trend in wave energy in the southern half of the east coast of Japan. Increased average wave period and height contributed to this trend on the east coast. We also found an increasing trend in surface wind speeds east of Japan. These results suggest that more frequent swells have been generating greater wave energy off Japan's east coast during the last decade.