What explains South Korean public opinion on nuclear weapons development? Despite the US security commitment, most South Koreans consistently support independent nuclear weapons development. To solve this puzzle, we investigate the mechanisms of security threat and security commitment under which individuals develop their opinion on nuclear proliferation. An original survey of 1,988 South Korean adults shows empirically supported correlates of South Korean public opinion on nuclear proliferation. Public support for proliferation is largely shaped by external security threats: nuclear threats from North Korea and China, but also the latent nuclear superiority of Japan. Public opposition to proliferation is fostered by the perceived credibility of the US security commitment, but not by its perceived effectiveness. The results provide insights into South Korean public opinion on nuclear proliferation and can inform the design of a nonproliferation policy tailored to South Korea.