On April 13, 2021, the Japanese government announced its decision to discharge the nuclear-contaminated water from the Fukushima Daiichi Nuclear Power Station into the ocean, which drew widespread opposition from the international community. Japan bears traditional state responsibility to prevent marine pollution from radioactive waste, which includes the obligation to immediately notify interested parties and implement preventive measures based on the precautionary principle. Further, Japan needs to fulfil the obligation to prevent transboundary harm, take measures to protect the marine environment from radioactive contaminants via various sources, and not to allow land-based discharge and dumping or any other form of discharge into the ocean. In the process of analysing the responsibility that Japan should bear, this paper discusses the potential contamination of nuclear-contaminated water and reached a conclusion that it should not be discharged into the ocean. To urge Japan to fulfil its international responsibilities, international cooperation among States should be strengthened to monitor the disposal of Japan's nuclear-contaminated water. In addition, getting advisory opinions from International Tribunal is a feasible method for relevant stakeholders. Moreover, it would be highly desirable if relevant treaties or organisations were created in order to better implement international environmental law. •Japan has the obligation to implement preventive measures to protect the marine environment from radioactive contaminants from various sources.•The nuclear-contaminated water discharged from Fukushima-Daichi nuclear power plant might affect the global marine environment.•Seeking for advisory opinion from International Tribunals and draft a global convention to regulate transboundary harm is also recommended.