In the last three decades Regional Parliamentary Institutions (RPIs) have experienced a rapid increase and spread across all regions around the globe. They represent a unique parliamentary phenomenon of international affairs that first and foremost exhibits a genuine legitimacy nexus between local constituencies and the international area. This paper builds on this characteristic and elaborates a legitimacy approach that identifies three legitimacy mechanisms that may help to conceptualize the establishment of specific design features of RPIs. To this end, a concise typology of RPIs with two disjunctive criteria – election mode and connection to a parent regional organization – provides the grounds for a systematic analysis of their organizational design. Building on a newly created dataset of 68 globally spread RPIs, the empirical analysis generates two main findings: (1) the rapid increase of RPIs after 1989 is empirically corroborated for all regions and most types of these institutions; (2) two standard applications of the developed legitimacy mechanisms – functional and normative legitimacy arguments – are not significant in explaining the choice of specific design features of RPIs. Therefore, the observed rapid increase and global spread of these institutions provide tentative evidence to support a diffusion analysis of their emergence and design, making the paper call for a more thorough conceptualization of RPIs’ organizational design and processes of inter-dependent decision-making.