This study examines the relationship between basic research and firm R&D productivity. In particular, this study focuses on the conditioning role of firm-level technological diversification in the relationship based on the idea that firms with a broad technological knowledge base are better positioned to fully harness the wide and unpredictable applicability of scientific knowledge. Using a unique panel dataset of Korean manufacturing firms during the period of 2003-2012, we find the following. First, firm-level technological diversification positively conditions the relationship between basic research and firm R&D productivity. In particular, unrelated technological diversification, rather than related technological diversification, has a positive conditioning effect on the relationship. Second, the positive conditioning effect of technological diversification is more pronounced for firms operating in high-tech industries or in less competitive industries. These results suggest that a broad technological knowledge base across unrelated technological fields is helpful or necessary for firms to better utilize basic research for enhancing their R&D productivity.