Since the beginning of the 21st century, the emergence of various institutionalization processes of new informal institutional spaces (such as BRICS, IBSA, BASIC or P5+1) has been observed internationally, but little has been studied in depth. Accordingly, this paper seeks to rethink, through a theoretical approach standing between Political Science and International Relations, the theoretical concepts of neo-institutionalism in order to make a new critical theoretical-analytical contribution that accounts for the particular characteristics of such phenomena. We believe that it is of interest for the study of international institutions to expand the current field of analysis in order to explain how the emergence of these new institutions occurred and how they interact with already established formal institutions. This article examines two traditional forms of neo-institutionalism (rational choice neo-institutionalism and historical neo-institutionalism), making explicit what the starting points of each are and what elements each can contribute to the study. At the end of the work, a dialogue is held between the perspectives that allow us to establish a theoretical analytical criterion to study these new phenomena.