Kotoku Shusui was born in a town which is now known as Kyomachi of Shimanto-city in the 4th year of Meiji (1871), and his real name was Denjiro.
His well-established family ran a pharmacy and sake brewery in the town. His father's name was Atsuaki and his mother's name was Taji. He was the youngest of their four children.

His father died while he was still young, so he was brought up entirely by his mother. Shusui was physically fragile and suffered from gastroenteric problems during his childhood, causing his family to doubt whether he would ever actually grow up to be a man. However, it is said that even an infant, he was so clever that he could write characters on his mother's chest with his fingers while he was being breast-fed. During his childhood, he was so intelligent and precocious that people called him a child prodigy. While attending elementary and junior high school, he was also able to attend Kido Kakusho's private school, where he started studying Chinese classics from the age of 9.

During his youth he started to take interest in the democratic rights movement, and started reading "Jiyuu Shimbun" (the Liberal Newspaper) at the age of 15. When he was 16 years old, he represented the townspeople by reading the congratulatory address at a small dinner party arranged for Itagaki
Taisuke's visit to Nakamura. He became Hayashi Yuzo's student houseboy the next year, but the government had already started to purge the liberal democrats. Next he became Nakae Chomin's student houseboy. Chomin was a person that Shusui respected and mentored throughout his entire life, and from whom he received a great deal of ideological and personal influence. Shusui became a newspaper writer, and as his reputation as a writer grew, he began gradually leaning towards socialism as he became into contact with various social problems.

The Japanese Social Democratic Party was formed in the 34th year of Meiji (1901), but was promptly forced to disperse in the same year. However, they found a way to continue their activities by joining forces with the Socialist Association. In the same year Shusui published his thesis entitled "The Problemof Imperialism in the 20th Century", which is a surprisingly insightful publication that demonstrates his pioneering theories 15 years before Lenin's "Imperialism".He advocated pacifism in the Russo-Japanese War of 1904-1905, and actively continued his work in the anti-war movement. In the 38th year of Meiji (1905), he was indicted for articles written while he was an editor of the Common People's Newspaper, and while in prison he read theses written by Engels and Kropotkin, which turned his political thoughts towards anarchism. After being released from prison, he traveled to the United States where he continued to be politically active, making speeches in Oakland celebrating the Russian Revolution and taking part in other political activities. His sympathy with the ideals of the Russian Revolution grew, which strengthened his views towards syndicalism. He predicted the Japan-US war in a paper he wrote, and throughout the time before the war continued to advocate the avoidance of war,as well as continuing to make pioneering insights that demonstrated his talents. On his return to Japan, Shusui argued the necessity of activism against parliamentarists at the 2nd Socialist Party's convention, an issue which finally caused a break of the party into two factions.

After the formation of the 2nd Katsura Cabinet, the crackdown on socialists was strenghtened and oppression became increasingly more severe. Shusui intended to retreat from the front line of the movement because of his financial hardship and poor health. Under the advice of his close friend, Koizumi Sanshin,he had devoted himself to a life of rest and writing at the Yugawara hot spring when he was arrested, accused of grand treason. He was put to death on the gallows, and his punishment was imposed by a closed trial, which was unprecedented in the legal history of Japan.

He died at 8:06 a.m. on January 24, in the 44th year of Meiji (1911), aged 40.The prison chaplain, Mr. Numanami, said that Shusui met his fate bravely, and remained stoic and undistracted until the end.