Product innovation, in particular radical product innovation, is considered to have a positive effect on company success. As a consequence, there is growing interest in the literature on how to foster radical product innovation. Organizational culture is frequently cited as an antecedent. The underlying assumption is that the culture facilitates radical innovation by encouraging employees' innovative behavior. However, empirical research on the mediating role of employees' innovative behavior in the relation between organizational culture and radical product innovation is scarce. The purpose of this paper is to analyze this mediating role using a sample of Spanish firms. Findings show that adhocracy and market cultures are positively related with the degree of radicalness of product innovation and that employees' innovative behavior mediates that relation, but only in the case of the adhocracy culture.