The regionalisation of the world economy is one of the most important developments in global governance in the past two decades. This process has seen ‘inter-regional’ economic agreements emerge between two or more regional groupings. Drawing mainly on the European Union’s external relations, observers accordingly point to the growing importance of regional actors, explaining their agency (or ‘actorness’) with regional attributes such as (supranational) institutional design, size, and member state cohesion. This article challenges this dominant explanation of regional agency. It argues that regional actors are socially, politically, and historically ‘embedded’. Agency reflects the contingency of regional integration processes, the motivations that underpin those processes, and the specific relationships between regions and third parties. This approach explains an important case of inter-regionalism from the Asia-Pacific: CER-ASEAN relations. Since the early 1990s, Australia and New Zealand have used their ‘Closer Economic Relations’ trade agreement for relations with the Association of Southeast Asian Nations. This reflects the ambitions of Australasian officials to shape processes of Asian-Pacific regionalism, and the interests of ASEAN officials in consolidating their own process of transnational market-making. Here, regional agency owed to a transforming world economy and the reconceptualisation of regions within new networks of trade governance.