Transnational organised crime at sea is a growing international concern. However, and despite its importance, the concept remains uncertain and contested. This ambiguity has led to a tendency to focus on individual challenges such as piracy or illegal fishing, rather than convergencies and synergies between and across issues, and has stymied a concerted international policy response. Debate continues over the term itself, what illicit activities it incorporates and excludes, and how these can be meaningfully conceptualised in ways that both recognise the diverse nature of the concept yet also provide a basis for an integrated response to the challenges it presents. In this paper, we address this lacuna by providing a systemic conceptualisation and analysis of transnational organised crime at sea. Our goal is to provide a firm basis for future enquiries on the different types of blue crime, to trace their distinct characteristics and identify how they intersect, and to consider what kinds of synergies can be built to respond to them. In so doing, we organise the nascent academic and policy discourse on blue criminology and maritime security to provide a new framework for navigating this complex issue for practitioners and analysts alike.