The Dark/Black Ocean Society (玄洋社 Gen'yōsha?) was an influential Pan-Asianist group and secret society active in the Empire of Japan, and was considered to be an ultranationalist group by GHQ in the International Military Tribunal for the Far East.
Founded as the Koyōsha by Hiraoka Kotarō (1851–1906), a wealthy ex-samurai and mine-owner, with mining interests in Manchuria, Tōyama Mitsuru, and other former samurai of the Fukuoka Domain, it agitated for a return to the old feudal Japanese order with special privileges and government stipends for the samurai class. The Koyōsha participated in the various ex-samurai uprising in Kyūshū against the early Meiji government, but after the suppression of the Satsuma Rebellion in 1877, it abandoned its original goals, joined the Freedom and People's Rights Movement, and formed a political organization to agitate for a national parliament instead.
In 1881, the Koyōsha changed its direction again. This time, the declared aims of the Gen'yōsha were "to honor the Imperial Family, respect the Empire and to guard the rights of the people". However, its true agenda was to agitate for Japanese military expansion and conquest of the Asian continent. The true agenda was reflected in its new name of Gen'yōsha, taken after the Genkainada strait which separates Japan from Korea.