Working collaboratively with suppliers is increasingly cited as a “best practice” in product development. The importance of sharing knowledge between buyer and supplier in this context has been well recognized, although comparatively little research exists on the interorganizational socialization mechanisms that facilitate it. The present research proposes and tests a theoretical model of the impact of formal and informal socialization mechanisms on the level of knowledge sharing within interorganizational product development projects and the subsequent effect on buyer firm performance. Results from this study of 111 manufacturing organizations in the United Kingdom largely support its hypotheses. It is revealed that informal socialization mechanisms (e.g., communication guidelines, social events) play an important role in facilitating interorganizational knowledge sharing, whereas formal socialization mechanisms (e.g., cross-functional teams, matrix reporting structures) act indirectly through informal socialization to influence knowledge sharing. The results also show that interorganizational knowledge sharing is positively associated with supplier contribution to development outcomes, which, in turn, improves buyer product development performance and, ultimately, financial performance. Product development managers are encouraged to build social ties between interorganizational development teams to increase the flow of knowledge and to improve both product development outcomes and financial performance.