This paper focuses on the issues of international law involved in discharging of nuclear contaminated water from a nuclear accident into the sea, taking Japan as an example, and proposes a pathway for marine environmental cooperation as a reference for regional States and the international community. Specifically, this paper first examines the current situation of the discharge of contaminated water from the nuclear accident in Japan. Secondly, we explore the negative effect of discharging contaminated water from the nuclear accident into the sea, particularly its impact on the marine environment and marine industries. Thirdly, we analyze the dilemma faced by the current system of international law in regulating the discharge of contaminated water from the nuclear accident into the sea. Fourthly, we illustrate the potential responsibilities of the NACW discharging States. Finally, the article takes international cooperation as the principal means and proposes several detailed ways to establish and develop the regulation regarding the discharge of contaminated water from the nuclear accident into the sea.