This article sheds new light, from a transnational perspective, on the intellectual struggle in Japan over nuclear deterrence. Japanese scientists opposed the Cold War order from the superpowers on down. Against the backdrop of the intensifying nuclear arms race between the United States and the Soviet Union, Pugwash scientists came to accept a key notion by the mid-1960s; namely, that stable mutual deterrence is a prerequisite for averting nuclear war and promoting nuclear arms control. Under such circumstances, the Japanese Pugwash scientists began to criticize nuclear deterrence in the early 1960s in Japanese society. This article recounts how their challenges to the intellectual hegemony of nuclear deterrence developed not only from antiwar and antinuclear sentiments that they shared with the Japanese public, but also from the transnational transfer of ideas through the Pugwash organization.