Since the Fukushima nuclear disaster of 2011, Japan has relied on imported fossil fuels for electricity generation, but is quickly increasing its share of renewable energy sources. Recent development has been in biomass, geothermal, wind, and solar PV, though little attention has been given to the potential of marine energy resources. The present paper carries out an analysis of the economic viability of this resource using real wave and tidal current data. The results show that marine energy technologies could be cost-effective at several locations in Japan, and can pragmatically add reliable and predictable power to the energy generation mix. Deployed in straits with strong tidal flows near large population centers in western Japan, SeaGen and Verdant-type tidal turbines are shown to operate at costs far below the current price of electricity in the country. In northern Japan, the Aquabuoy, Pelamis, WaveDragon, and Guarda-type Oscillating Water Column Wave Energy Converters show costs near the current price of electricity. Even though Aquabuoy and Pelamis are now defunct, it is likely that in the future new generation wave energy converters would be able to produce electricity at even lower costs and further enhance the practicality of developing wave power in northern Japan.