Eco-innovation is an important component of a firm's environmental sustainability strategy and provides both environmental benefits and competitiveness, resulting in a win-win situation. Although previous studies have examined the influence of organizational ambidexterity (exploration and exploitation) on the business performance, little is known about how organizational ambidexterity affects eco-innovation (i.e. eco-process, eco-product, and eco-organizational innovativeness). Building upon the resource-based view and dynamic capabilities perspective, I develop a model to explain how relational capital may moderate the relationship between inter-firm relational knowledge stores and organizational ambidexterity and how organizational ambidexterity enhance eco-innovation in the context of international buyer-supplier relationships. The results of a survey of 124 OEM suppliers in Taiwan show that the relational knowledge stores have a positive effect on organizational ambidexterity, and organizational ambidexterity has a positive effect on eco-innovation. Our findings also show that the direct relationship between relational knowledge stores and organizational ambidexterity was stronger when the relational capital was high as opposed to when it was low. This study contributes to a theoretical understanding of why some firms engage in organizational ambidexterity than others, analyzing relational knowledge stores as a predictor and relational capital as moderator. I discuss the theoretical and managerial implications of our findings.