Research on environmental justice in authoritarian regimes, and in particular on how transnational networks support problem framing and claims-making in the absence of state-led democratic participation instances, is limited. This article uses the case of untreated wastewater from a steel mill owned by Taiwanese conglomerate Formosa Plastics Group, which caused mass fish deaths along coastal provinces in Vietnam in 2016, to explore how civic groups and local communities problematize official accounts of events and engage with transnational networks to make claims to environmental injustice. The paper highlights local narratives about the adverse impacts of the disaster on residents’ livelihoods and wellbeing, controversies over the causes of and responsibility for the disaster, and the role of transnational alliances with Taiwan in sustaining and magnifying claims to injustice. We argue that viewing issues such as the Formosa steel incident through a transnational environmental justice lens illuminates the effect of global and national processes of economic reform in shaping uneven environmental and social impacts from new infrastructure developments. We also argue that thinking in terms of transnational networks can make sense of the spaces which can emerge for claims-making in authoritarian contexts, where democratic participation instances and access to knowledge may be restricted.