One of the most prominent aspects of the 2011 conflict in Libya was the overt support, both military and non-military, offered to the Libyan anti-Gaddafi rebels by the States that intervened in the conflict. The present article evaluates the conformity of this support with the rules of jus contra bellum and jus in bello. From a jus contra bellum perspective, support of the Libyan rebels exceeds the ‘necessary measures’ that the intervening States were allowed to take in order to protect the civilian population in the Libyan conflict according to Security Council resolution 1973 (2011). From a jus in bello perspective, instead of identifying possible violations of international humanitarian law during military operations on a case-by-case basis, the article takes a step back and analyses the legality of the support of the rebels as such. In view of the violations of humanitarian rules reportedly committed by the rebels, the continuous support of the rebels constitutes, on behalf of the supporting States, a violation of the customary obligation to ensure respect for international humanitarian law.