Socialist leader, one of the first proponents of radical political action in Japan. His execution resulted in the temporary abatement of the growing Socialist movement in Japan.

Of relatively humble origin, Kōtoku started work as a houseboy in the Tokyo home of Hayashi Yūzō, one of the most famous liberal politicians of his day. He obtained an education and in 1893 became a newspaper writer. One of the earliest advocates of Socialism in Japan, Kōtoku helped organize the Social Democratic Party in 1901. The party was immediately banned by the government, however, and Kōtoku, together with Sakai Toshihiko, then began his own newspaper, the Heimin shimbun (“Commoner's Newspaper”). After it opposed the Russo-Japanese War (1904–05), the paper was closed and Kōtoku was imprisoned. Released after five months, he toured the United States, where he was impressed by the direct action policies of a radical U.S. labour group, the “Wobblies” (Industrial Workers of the World).

Returning to Japan, he denounced parliamentary politics and began to organize workers for radical activities. This movement was crushed, however, when in 1910 hundreds were arrested on charges of being involved in a plot to assassinate the Emperor. Although Kōtoku had withdrawn from the conspiracy, and at the end only four men were shown to be actually involved, Kōtoku was included among the 11 who were imprisoned and subsequently executed. The backlash resulting from the plot ended the Socialist movement as a major force in Japan for almost a decade. Kōtoku spent the last months of his life writing articles denouncing Christianity.