The onset of nuclear warfare in Hiroshima and Nagasaki had far-reaching implications for the world of medicine. The study of the A-bomb and its implications led to the launching of new fields and avenues of research, most notably in genetics and radiation studies. Far less understood and under-studied was the impact of nuclear research on psychiatric medicine. Psychological research, however, was a major focus of post-war military and civilian research into the bomb. This research and the perceived revolutionary impact of atomic energy and warfare on society, this paper argues, played an important role in the global development of post-war psychiatry. Focusing on psychiatrists in North America, Japan and the United Nations, this paper examines the reaction of the profession to the nuclear age from the early post-war period to the mid 1960s. The way psychiatric medicine related to atomic issues, I argue, shifted significantly between the immediate post-war period and the 1960s. While the early post-war psychiatrists sought to help society deal with and adjust to the new nuclear reality, later psychiatrists moved towards a more radical position that sought to resist the establishment’s efforts to normalise the bomb and nuclear energy. This shift had important consequences for research into the psychological trauma suffered by victims of nuclear warfare, which, ultimately, together with other research into the impact of war and systematic violence, led to our current understanding of Post-Traumatic Stress Disorder (PTSD).