Regional security institutions are founded on security systems, such as cooperative security, collective self-defense, and collective security, which play an important role in ensuring member states’ survival in an anarchic international environment. However, such a materialist perspective tells only a partial story about the roles of regional security institutions; these entities also help to define member states’ cognitive template of regional security. Given the concepts of “region” and “security” are both socially constructed, “regional security” is the concept that is ultimately defined and redefined by the group of states, and thus inter-state regional security institutions play an imperative role in that process.

How do regional security institutions shape the concept of regional security? The most relevant theoretical framework to answer this question is the Copenhagen School’s “securitization” theory. It offers analytical insights in understanding the social construction process of regional security by emphasizing important factors such as speech acts, audience acceptance, and extraordinary measures. Nevertheless, its recent research focus has deviated from inter-state relations and regional security institutions, and the question has been left unanswered.

In this context, this article, employing a synthesized analytical framework of securitization theory and an agent-centered historical institutionalism, argues that regional security institutions become a “securitization” tool for member states by providing them a staged process to collectively define their own regional security. Specifically, this article proposes a two-stage hypothesis. First, the member states’ perception of a change in the regional distribution of power triggers the securitization process. Second, the securitization process will be completed if member states accept a new threat perception by a securitizing actor and conduct extraordinary measures to deal with the threat. These measures can be conducted only when the existing institutional function cannot manage the new threats. After this cognitive template of regional security is consolidated within the institution, it begins to constrain member states’ strategic thinking and choices.

To test the hypotheses, this article conducts a comparative case study with the Association of Southeast Asian Nations (ASEAN)—in the process of establishing the ASEAN Regional Forum in 1994—and the Economic Community of West African States (ECOWAS)—in the process of establishing the protocol relating to the mechanism for conflict prevention, management, resolution, peace-keeping and security.

The basic findings are three-fold. First, member states’ perception of a change in the regional distribution of power mattered in setting off the securitization process. Second, member states’ agreement with new security threats and extraordinary measures are imperative to complete the securitization process, but a securitization actor can be varied in at each step of the process. Third, historically embedded norms of the regional security institutions should be taken seriously as it narrows the range of strategic choices on extraordinary measures. Despite a lack of sufficient number of case studies, the ASEAN and ECOWAS cases revealed that the complex securitization processes through regional security institutions created and recreated a cognitive template of regional security, which framed their security perspectives.