This article examines the viability of integrating the duty to rescue concept in international criminal justice to deal with the responsibility of bystanders. Despite the fact that they often contribute to create the social context in which mass crimes occur, bystanders are almost absent from the scope of international criminal justice, focusing mainly on the perpetrators and the victims. This article explores a possible avenue to fill this gap so that the attribution of responsibility for mass crimes can be more coherent with their collective dimension. It assesses whether the duty to rescue concept, commonly found in the legislation of civil law countries, could provide a ‘ready-made’ solution to deal with bystander responsibility. Following a comparative analysis of the different approaches to the duty to rescue in civil law and common law countries, it examines how the duty to rescue would fit with similar concepts in international criminal law.