Knowledge, as one of the most important resources, has a strong impact on inter-organizational relationships, particularly in key account (KA) relationships where knowledge sharing is essential. Previous research has shown that customer-specific knowledge is often tied to the respective KA manager and that knowledge sharing is especially complex, which raises the challenge of how to foster knowledge sharing to increase KA performance. However, the current state of research lacks an integrative perspective that considers both organizational and individual factors as relevant for knowledge sharing and KA performance. The research approach is twofold: First, a qualitative study based on 88 semi-structured interviews investigates the role of organizational factors and the individuals' role in knowledge sharing. Second, a quantitative moderated mediation analysis reinforces these associations using survey data from 307 respondents. The results show that identification with the company positively influences knowledge sharing behavior and that high knowledge sharing behavior has a positive influence on KA performance, which is moderated by the degree of formalization. The results of the analyses contribute to the research of KA management and knowledge management and provide new insights by focusing on both, organizational and individual factors and by applying assumptions from social identity theory.