One of the most striking questions raised by Brexit is undoubtedly related to security and defence. What should we expect from the United Kingdom (UK) in the collective and multilateral efforts within the European Union (EU) and North Atlantic Treaty Organisation (NATO) after Brexit? This article aims to contribute to an answer by arguing that, in the security and defence field, Brexit hardly represents a threat to NATO or the EU. Instead, it offers an opportunity to readjust priorities and investments towards an effective multilateralism, both European and transatlantic. The paper argues that, faced with an apparent overlap of the threats and challenges shaped by the strategic environment and the priority concerns of both organisations, it is expected that the UK will continue to be engaged in the EU common security and defence policy’s (CSDP) civilian missions and military operations as a third country, as well as enhanceNATO’s commitments in the aftermath of the implementation of the 2016 Warsaw Declaration.