Early influenced by Christianity, Masamune went to Tokyo in 1896 to enter Tokyo Senmon Gakkō (later Waseda University); he was baptized the following year. In 1903 he began writing literary, art, and cultural criticism for the newspaper Yomiuri. The novels Doko-e (1908; “Whither?”) and Doro ningyō (1911; The Mud Doll) brought him attention as a writer of fiction, although he was already known for his distinctive criticism. These are stories of people living in a gray world devoid of all ambition and hope; Ushibeya no nioi (1916; “The Stench of the Stable”) and Shisha seisha (1916; “The Dead and the Living”) are similar works. Masamune also devoted some time to writing plays, the best known of which is perhaps Jinsei no kōfuku (1924; “The Happiness of Human Life”).

It is in criticism that Masamune is often considered to have done his best work. In 1932 he published the influential Bundan jimbutsu hyōron (“Critical Essays on Literary Figures”). Other outstanding critical works are Shisō mushisō (1938; “Thought and Non-Thought”) and Bundanteki jijoden (1938; “A Literary Autobiography”).