Japan adopted a series of policies in 1971 that came to be known as the ‘Three Non-Nuclear Principles’, outlining that Japan will not produce, possess, or introduce nuclear weapons. The Three Non-Nuclear Principles featured an exceptionally high degree of non-proliferation commitment when compared to other states, and they have remained the backbone of Japanese nuclear policy to the present day. Then, what factors motivated Japan to adopt a set of unique non-nuclear policies such as the Three Non-Nuclear Principles in 1971? To answer this question, this paper examined the normative background of the Three Non-Nuclear Principles, with an emphasis on the norms that motivated the domestic actors in Japan at the time. The findings show that the following four distinct norms in Japan motivated the domestic actors to advocate the adoption of the Three Non-Nuclear Principles in 1971: 1) Japan’s non-nuclear norms, 2) Japan’s anti-war norms, 3) regional norms in Okinawa, 4) conservative norms in the ruling Liberal Democratic Party’s leadership.