We examine the role of internal and external relational social capital (SC) as determinants of radical product innovation (RPI). By analysing both sides of SC, we provide interesting insights on their relative influence and their interaction effect on this type of innovation. Besides, traditional assumptions on SC and innovation are questioned. In our empirical study using a sample of 142 manufacturing and service companies we found that, of the two types of SC analysed, internal SC is the most relevant predictor of RPI in relation to either technological or market dimensions. The influence of external SC is not as strong as the internal one. Regarding their interaction effect, external SC reduces the positive effect of internal SC on the market dimension of RPI. Interesting implications arise for practitioners, who should pay special attention to the higher impact of internal SC on RPI and the need to carefully manage the difficulties that emerge when it is combined with external SC.