This article explores the diplomatic implications of a central pillar in EU external relations: the development of interregional relations. In particular, the article investigates the emergence of a specific pattern of interregional relations — 'complex interregionalism' — and develops an initial framework for the analysis of this phenomenon. This framework allows for a detailed investigation of how the EU has simultaneously engaged in bilateral, multilateral and interregional relations across the globe. The EU — notably the Commission — is found to have a consistent and coherent complex interregional strategy that it employs across three world regions: Asia; Africa; and Latin America. This strategy embodies multi-level interregional relations, but aspires to the creation of 'pure interregionalism' between the EU and other customs unions. Such a strategy presents two key tensions that lie at the heart of 'complex interregionalism': the first tension is between the reality of multi-level diplomacy and the desire for 'pure interregionalism'; and the second is between the Commission's strategic vision and the realities of Council-shaped diplomacy. Analysis of the internal and external dynamics of the strategic pursuit of interregionalism, and the failure to implement it fully, can thus offer important insights for the study of both the EU's external relations and EU diplomacy.