In the construction of the European Union's (EU's) counter-terrorism legal system, United Nations instruments and principles enter in the EU law sphere and pave the way for further developments both due to its legal force and to its role as agents of socialisation of security norms. At the same time, as this article demonstrates, the principles of EU constitutionalism, fundamental pillars in the EU's legal system, have been the sources of judicial decisions that ultimately impacted on the standards of fundamental rights protection in the UN framework. This article investigates the mutual influences between the EU and the UN in the field of counter-terrorism. It argues that although the impact of UN norms on EU counter-terrorism policies is more significant than the opposite, the EU has also revealed capacity to influence the UN in certain counter-terrorism procedures. Considering the sensitivity of counter-terrorism and EU's characteristics as an external security actor, the findings of the research conducted here can be seen as counter-intuitive, enabling new discussions on the EU's security actorness.