Science diplomacy represents a relatively novel field of EU external action. This contribution provides a better understanding of the under‐researched development of this policy domain, asking how and why the EU has extended the scope of its external activities in the scientific area. To answer these questions, it conducts an analysis of EU policy‐makers' discourses on external science policies inspired by role theory. It finds that EU role conceptions in this domain take the forms of ‘science for diplomacy’ or ‘diplomacy for science’, and that they correspond either to an image of normative or market power Europe. These findings are confirmed by a study of the EU's recent external science policies. The article argues that the ambiguity of rationales invoked for introducing a genuine science diplomacy in the EU helps to appeal to different constituencies. This, in turn, enhances the Union's chances for successfully expanding the scope of its activities.