Relationship management plays a critical role in sustainable supply chain management (SCM). The literature focuses on the operational and strategic levels of inter-organisational relationships in supply chains, where little is known about the psychological aspect of these relationships. Adopting social identity theory as the key theoretical lens, this research investigates inter-organisational relationships in the context of sustainability implementation in supply chains. Two research questions are asked: RQ1 How do focal organisations engage their supply chain stakeholders in sustainable SCM using social identity thinking? RQ2 What are the specific identity issues relating to inter-organisational relationships in a sustainability context? This research adopts an exploratory case approach and combines multiple data sources: semi-structured interviews, participant observation, and secondary data analysis. Three case studies are conducted in three international organisations and their suppliers in a global setting. The research findings reveal that organisations face the challenge of managing multiple identities during their sustainability implementation in supply chains. Subject to their operational context and supply chain characteristics, organisations may have different approaches to sustainability implementation. Sustainability identity is associated with internal stakeholders’ motivation and proactivity when contacting external stakeholders, as well as external stakeholders’ commitment to cooperation and information sharing in sustainable supply. Analysis of the 41 interviews reveals that both the internal and external stakeholders play an important role in identity formation through various identity assessment and comparison activities. The key identity issues observed during sustainable supply practices include identity conflicts, inconsistencies, and disagreements among the stakeholders. The current SCM literature focuses on supply chains as one uniform identity. A major contribution of this research is that it addresses the complexity of identity issues in supply chains, especially in the context of sustainability implementation. It also contributes to theory by defining a typology of focal organisations’ identity regulation approaches.