Purpose: The purpose of this paper is to propose and empirically test intellectual capital (IC) as a mediator in the corporate social responsibility (CSR) and financial performance (FP) relationship. Design/methodology/approach: The empirical research was conducted on 345 European firms listed in the STOXX Europe 600 index. To evaluate the mediating effect of IC, we applied the four-step Baron and Kenny model, tested through an ordinary least squares regression analysis. Findings: The findings highlighted a partial mediation of IC on the CSR–FP relationship, suggesting that the implementation of CSR strategies has a positive effect on the development of firms' IC, which in turn enhances firms' competitive advantage and superior long-term FPs. Originality/value: We found a new mediator in the CSR–FP relationship and we contribute to a new line of research that aims to study environmental and sustainability aspects strictly interrelated with IC and performances (sustainable intellectual capital).