Ernst Rüdin (1874–1952) was one of the founders of psychiatric genetics. From 1931 until 1945, he was Director of the German Institute for Psychiatric Research (Deutsche Forschungsanstalt für Psychiatrie, DFA) and from 1935 until 1945 President of the German Association of Neurologists and Psychiatrists. The historical knowledge on Rüdin is reconsidered, using new sources, and with a focus on Rüdin’s involvement in the Nazi program of patient killings („euthanasia"). It is documented that (1) it is a misconception to interpret psychiatric genetics as „instrumentalised" by the Nazi regime; (2) Rüdin belonged to the group of psychiatric perpetrators who worked to give the Nazi selection policies scientific authority; (3) Rüdin knew early on about the patient killings and refused to support others in their efforts to stop them; and (4) the research in the context of the patient killings carried out by Julius Deussen, a member of Rüdin’s team at the DFA, was initiated and actively supported by Rüdin, and co-financed by the DFA.