High-tech manufacturers increasingly rely on the knowledge contributions of external technology experts (ETEs), who contribute to collaborative R&D projects on behalf of suppliers. Many scholars have considered knowledge sharing in R&D collaborations from a firm-level or project-level perspective and focused on formalization as a potential remedy. While individual supplier employees at the operative level make the decision to share critical knowledge, the individual-level perspective in literature on knowledge sharing in collaborative R&D projects is virtually nonexistent. Because knowledge sharing in collaborative R&D is a largely discretionary act on behalf of the supplier employee, personal motivations rather than inter-firm relationship elements (e.g., network position or dependency) become the primary determinant of one’s sharing behavior. Abstracting from or ignoring these motivations of supplier employees in studies on collaborative R&D may obscure important insights for R&D managers. This study is an important first step in providing the empirical evidence needed to uncover the motivational and behavioral foundations for ETEs’ knowledge sharing in a collaborative R&D setting. Building on theories of gift and social exchange, this article identifies customer stewardship and distributive fairness as two important personal motivations of ETEs to share knowledge. Project formalization is considered as a key contingency condition. Analyzing survey responses of 186 ETEs, a multilevel regression-based moderated-mediation analysis of direct and indirect effects shows that customer stewardship predicts an ETE’s knowledge sharing behavior under (very) low levels of project formalization, and distributive fairness predicts knowledge sharing behavior under medium to high levels of formalization. Together, the results provide R&D project managers who aim to leverage external knowledge contributions with valuable insights that have been obscured in past firm-level collaborative R&D studies.