This article seeks to address the European Union's (EU's) role as an interorganizational influencer vis-a-vis regional organizations. More specifically, it examines and assesses the extent to which the EU has been able to shape the institutional designs, policies and practices of the African Union (AU) and the Association of Southeast Asian Nations (ASEAN) in the security domain. Both organizations are regional of nature, possess specific tasks and responsibilities which include the realm of security, and maintain interorganizational relations with the EU. While the EU has been heavily influenced by international organizations and international institutions, it also has the potential itself to exert interorganizational influence. This potential can put the EU in different positions as interorganizational influencer. These positions vary from role model and to a limited influencer. It is argued that in the early stages of the EU's interorganizational relations, its ability to shape the evolution of its counterparts is greater than once the relationships have been strengthened because of the EU's tools and resources. The case examples of the AU and ASEAN will serve to illustrate the varying degree of the EU's influence on international organizations. © 2018 Kluwer Law International BV, The Netherlands.