Despite the rise of substates as international actors, theoretically informed frameworks lag behind the expansion in substate diplomacy. Dedicated attention to the cultural dimensions of their international activity has also been limited. Based on examining the case study of Wales, this article advances a historical institutionalist analysis to explain how substate governments engage and interact in international cultural relations. It highlights the strengths of Lecours’ framework with its emphasis on the multilevel institutional structures and structure-agency dynamic conditioning substate diplomacy. The article adapts and updates the framework and offers an expanded historical institutionalist analysis by pointing to the value of understanding longer-term institutional and policy contexts and the need for greater attention to the role of ideas in explanations of substate diplomacy. On this basis, the article makes the case for the broader applicability of this historical institutionalist approach to analyzing substate diplomacy in other cases in both federal and regionalized states.