In the field of international criminal justice, Rapoza has served on two United Nations war crimes tribunals and is involved in a number of international legal initiatives. From 2003 to 2005 he took an unpaid leave of absence from the Massachusetts Appeals Court to work as an international judge and coordinator of the war crimes tribunal set up by the UN to prosecute crimes against humanity and other serious offenses committed during the Indonesian occupation of East Timor. On this tribunal he was the only judge from a common law country and the only one who had also worked as an attorney which gave him a unique perspective on the proceedings. In 2012, he was appointed as a reserve judge on the Supreme Court Chamber of the UN-backed Khmer Rouge Tribunal in Cambodia. He also led a UN Criminal Justice Advisory Team in Haiti.