Purpose This study aims to examine the effects of external knowledge sourcing breadth and depth and explore whether there exists a complementary effect of breadth (or depth) and knowledge management (KM) capacity on firms’ innovation performance. Specifically, this study investigates the direct effects of breadth, depth and KM capacity on product and process innovation outcomes and tests whether complementary effects exist between breadth and KM capacity and between depth and KM capacity. Design/methodology/approach This study uses the survey data of 302 manufacturing firms in Thailand and uses ordinal regression analysis to test the hypotheses empirically. Findings Regression results reveal that breadth and KM capacity are essential for enhancing firms' innovation performance, while depth is not significant. However, the authors do not find a significance of complementary effects between breadth and KM capacity and depth and KM capacity on firms’ innovation performance. Originality/value This study provides additional evidence to contribute to an ongoing debate on what knowledge sourcing strategies (breadth or depth) are significant for enhancing firms’ innovations. Moreover, it explores whether complementary effects between KM capacity and breadth/depth exist in determining firms' innovation performance, which is a neglected issue in the existing literature.