Extant research on the relationship between firm complexity and corporate social responsibility (CSR) is limited to employing size variables as indicators for firm complexity. Following a contingency approach and to fit the multidimensional nature of CSR, in this study we define complexity as the structural complexity of a firm. To answer the question of whether firm complexity influences CSR, we study firms included in the major world stock indices for a 10-year period by means of a fixed-effects regression. Findings generally suggest a higher level of CSR in more complex firms. A more detailed analysis of complexity points toward a significant positive influence of the vertical, functional, and occupational dimensions of complexity but finds no link between spatial differentiation and CSR. This research represents the first empirical study that examines the relationship between CSR and structural complexity beyond a simplistic proxy of size.