This article comprehensively discusses the maritime dimension of the European Union's (EU's) security, which encompasses military and civilian aspects, intergovernmental and community components as well as institutional and geopolitical elements. First, the article provides a narrative of the development of the maritime element in the EU's security policy since the adoption of the European Security Strategy in 2003. By depicting the interrelations between the sea and the EU's security, the article shows that the maritime dimension of EU security is generally well established, but often obscured by the complicated institutional structure of the Union. Thereafter, the article emphasises the need to define an effective EU Maritime Security Strategy, which would provide a strategic framework for the Union's security-related activities regarding the sea that encompass maritime power projection, as well as maritime security and safety. Accordingly the article provides some recommendations concerning the definition of such a strategy and for appropriate constituting elements: the maritime-related risks and threats, the maritime strategic objectives, the means to implement the strategy, and the theatres of EU maritime operations.