governance actions in countries. One of the most important forms of such regimes is international indices, which compare and rank countries and, by generating cognitive and media pressure, exert significant effects on policymakers’ motivations and decisions, national policies, and public opinion. This study aims to investigate the mechanisms through which one particular transnational performance measurement regime, namely the Happiness Index, affects the governance system in Iran. In terms of its purpose, the study is fundamental, and methodologically, it adopts a qualitative research approach. Process tracing is employed to examine the mechanisms through which the index influences the governance system. The research population comprises all written and electronic documents, interviews, and documented actions of political, social, and media actors. The findings indicate that the introduction and dissemination of this index have influenced governance actions in Iran by inducing governmental and non-governmental actors to respond and take action, while also shaping governance priorities and measures. The transnational index has contributed to changes in governance behavior in several domains, including media discourse, political parties’ agendas, governmental officials’ responses, the division of responsibilities among national institutions, the production of relevant data and statistics, and the shaping of public opinion, among others. The study therefore recommends adopting informed response strategies and forward-looking approaches when engaging with transnational performance measurement regimes.