Since the adoption of the first United Nations Security Council (UNSC) counterterrorism resolution after the 9/11 attacks, the UNSC has increasingly required the domestic criminalization of “terrorism” acts and ancillary activities. Without the inclusion of an explicit international humanitarian law (IHL) or humanitarian exception, the UNSC has – so far – failed to harmonize the counterterrorism legal framework with IHL, leaving it up to States to define the interaction between the two. In their national legislation and courts, States’ interpretations have varied but counterterrorism legislations have been used to adjudicate conducts in armed conflicts, regardless of their legality under IHL. As the domestication of UNSC offences is ongoing, good practices are highlighted in this paper and recommendations are offered to ensure the development of international customary law in accordance with IHL.