To protect migrant fishers, Thailand ratified the International Labour Organization (ILO) Work in Fishing Convention in January 2019. This adoption of international norms was puzzling, as it occurred despite widespread protests from the most influential stakeholder: regional and national fisheries associations. Existing International Relations approaches to norm diffusion fail to explain this phenomenon. The norm socialisation approach expects Thailand's decision to be shaped by peer pressure from ILO members and yet such pressure was not exerted. By comparison, the norm localisation approach anticipates the ratification with local actors' support and yet domestic constituents staunchly opposed norm adoption. Both perspectives are therefore inadequate to explain Thailand's behaviour in relation to the ratification. In contrast, Auethavornpipat argues norm contestation is central to understanding the Convention's adoption in Thailand. Applying the critical norm approach, Auethavornpipat argues that domestic opposition is a normal and integral process whereby 'affected stakeholders' negotiate and re-negotiate norms' intersubjectivity. By tracing the activities of involved actors, I show that through contestation, stakeholders deliberated on the shared validity of norms that eventually resulted in the acceptance of the Convention. This lecture further illustrates that the contestation validated international norms domestically; however as opposed to the critical approach’s expectation, it has weakened the robustness of migrant worker rights in Thailand.