This article examines the redress campaign waged by activists in Japan on behalf of roughly 2,000 North Korean A-bomb victims (pipokja). These victims were repatriated from Japan after being subjected to the 1945 US nuclear attacks on Hiroshima and Nagasaki, while under colonial rule. From the early 1990s through to the twenty-first century, activists in Japan pursued redress for these A-bomb survivors in close synchronicity with the redress movements centred on South Korean victims. Highlighting the potential of the individual as entrepreneur within collective action settings, the redress developments were initiated and largely driven by an activist, Lee Sil-gun (1929–2020). Although Tokyo and Pyongyang were initially reluctant to acknowledge that A-bomb survivors existed in North Korea, in the face of sustained pressure by the Japan-based activists, the two governments facilitated a limited redress process for the victims by making various concessions on the issue. How did these activists navigate the structural constraints of the authoritarian North Korean state and the volatile bilateral relationship in enacting their transnational activism? How were they able to elicit concessions on their redress objectives from Tokyo and Pyongyang in the absence of formalized diplomatic relations? Drawing on fieldwork conducted in Japan and South Korea, this article probes these questions by empirically tracing and analyzing the evolution of the redress campaign for the North Korean A-bomb victims. I utilize the concept of polylateral diplomacy to elucidate the dynamic of engagement between the activists and the two governments.