This paper aims to denaturalize the norm that Japanese first-person pronouns are gendered by analyzing why and how this norm was constructed from a historical discourse perspective. Focusing on two time periods, the early modernization era of the late nineteenth century and the wartime era of the first half of the twentieth century, the paper analyzes metapragmatic discourses found in the writings of linguists and intellectuals and in grammar books and school readers. Four main findings emerge, all of them related to the sexist nationalist ideologies that defined both periods. First, in the modernization era, nationalist ideologies promoted the construction of a national language, of which males were assumed to be the primary speakers. Second, this assumption legitimated linguists' practices of including masculinized pronouns and excluding feminized pronouns from the national language, such that gender-neutral watakushi and watashi and masculinized boku became the standard first-person pronouns, while no feminized pronoun was considered a standard form. Third, in Japan's linguistic colonization of East Asia during the wartime era, linguistic gender differences were highlighted as evidence of the superiority of the Japanese language. Fourth, to emphasize such linguistic gender differences, females' use of the masculinized pronoun boku was proscribed, while the norm of distinct gendered pronouns—especially watashi for females and boku for males—was gradually confirmed. These findings illustrate the importance of examining metapragmatic discourses from the diachronic perspective to deconstruct naturalized linguistic norms.