This study explores the effect of board interlock on the diffusion of corporate social responsibility (CSR) among firms. While existing CSR literature has theorized a variety of institutional and organizational factors that drive CSR, what is lacking is a deeper understanding of how meso-level inter-firm relationships shape firms’ CSR. Using a series of pairwise regressions to examine Chinese listed firms’ CSR activities from 2008 to 2015, we find that board interlock leads to a higher probability of CSR adoption in both firms, and convergence of CSR structures. This interlock effect is however conditioned by some institutional and firm-level factors that enhance/impede the exchange of information between interlocked firms. This study contributes to the CSR literature by filling the theoretical gap on a meso-level mechanism of CSR diffusion and provides some managerial implications for corporate executives who are responsible for designing firms’ social programs.