Using previously uncited archival evidence from the US, the UK, and Czechia, we highlight Bohuslav Ečer’s significant impact on the Charter of the International Military Tribunal held at Nuremberg from 1945 to 1946. A politically motivated jurist from Czechoslovakia, Ečer proved a key innovator in international criminal law, pushing for bold new precedents at Nuremberg. We highlight multiple legal arguments that Ečer innovated and championed. These include the prosecution of individuals for the crime of aggression, the forfeiting of sovereign immunity during wartime, a broader definition of war crimes that included “crimes against humanity,” and the collective responsibility of certain “criminal” Nazi organizations as a means of streamlining individual-level prosecutions. We trace Ečer’s political thought and activism, including how his arguments joined with those of other prominent legal thinkers outside the US, including Hersch Lauterpacht and Aron Trainin. The article thus adds an important, yet often overlooked, voice to Nuremberg’s intellectual history and helps remedy the Anglo-American bias of dominant histories of international criminal law.