This work studies the linkages between spatially bound knowledge spillovers, internal research, and innovation (R&I) activities and firm productivity. Spillovers are modeled to emanate from intra- and extra-sectoral R&I activities in the firms’ regional business environments. We specifically test for non-linearities in the complex relationship between these internal and external knowledge sources and quantify their joint marginal effect on firm productivity. Our empirical results for a large panel of German manufacturing firms (1) underline the overall importance of knowledge spillovers in driving productivity and (2) point at distinct interactions between the included knowledge sources: First, we find that intra-sectoral knowledge spillovers only have a statistically significant effect on firm productivity when extra-sectoral spillovers are sufficiently large. Secondly, the link between knowledge spillovers and productivity varies with the level of the firms’ internal R&I activities.