Following the atomic destruction of Hiroshima and its occupation by Allied forces, Hiroshima city elites presented the rebuilding of their city as a symbolic enterprise for the cause of peace. Facing severe censorship, and in need of funds, Hiroshima and Nagasaki embraced a discourse of transformation focusing not on grief but on reconciliation and hope. With the emergence of the antibomb movement, this message expanded beyond the stricken cities. This discourse obtained special potency through the testimony of hibakusha (bomb survivors). This paper examines the origins of testimony in Hiroshima as a social practice and the use of survivor testimony by antibomb groups to galvanize support. Using “emotional mobilization,” the antibomb movement brought together private pain and public speech and turned the suffering of hibakusha into political capital. This move had wide, though not unanimous, support among the survivor community. Similarly to the later AIDS movement and the use of testimony by Zionist groups in the sixties following the Eichmann trial, political organizers in mid-fifties Hiroshima told hibakusha “do not mourn, organize.” This positive message led many to find meaning in the movement to gain compensation and work for peace and had important implications to the rise of the practice of bearing witness and later practices of transnational justice. Such practices are usually thought of in connection with Western traditions and historical trajectories but as the Japanese example shows they were in fact a transnational development with varied roots.