PurposeThe objective of the study is to explore legitimation dynamics in a public–private integration process and to gain insights on the specific role of CSR in triggering public–private logics.Design/methodology/approachCorporate social responsibility (CSR) is part of firms' strategy in gaining legitimacy from their stakeholders in a merger context. However, little is known about the role of CSR in triggering diverse dynamics from public or private logics during post-merger integration. This study aims at exploring the specific role of CSR in triggering such diverse logics. A qualitative research design based on a single case study of a public–private merger of two French listed companies in the urban planning sector was opted for. The analysis was pursued in real time from the signing of the agreement and then over two years.FindingsThe results show that public–private legitimation is a process that proceeds in stages. The authors emphasize the key factors that characterize it: align on external concerns: reflecting societal and institutional pressures (public legitimation); readapt to make sense internally in relation to the merger through managerial innovation (private legitimation) and CSR as a form of corporate self-storying: combining the social and societal aspects of CSR within the organization (hybrid legitimation). Three major actions were identified in activating a CSR legitimation strategy: identifying and responding to local needs; building a unified brand, culture, and employee commitment to the organization; and creating sustainable programs.Research limitations/implicationsThe first major contribution is linked to triggers influencing legitimation dynamics and in particular the role of CSR operating as a legitimation strategy in the merger integration process. A second theoretical contribution is linked to the evolutionary nature of the post-merger integration process. The processual study shows how stakeholder legitimacy demands can escalate and change over time.Practical implicationsFirst, three major actions were identified as key steps in activating a CSR legitimation strategy (identifying and responding to local needs; building a unified brand, culture, and employee commitment to the organization; and creating sustainable programs). These missions can be understood as key steps for managers in implementing CSR within an organization in a post-merger integration context. Second, this study increases our comprehension of legitimation as a dynamic micro-process. The different stages described in the study can be considered by the managers involved in the merger process as learning experiences to understand the complex phenomenon that is the integration process.Originality/valueThis study enriches the legitimacy-as-process perspective in providing insights on the specific role of CSR in triggering public–private logics.