Background: Digital platforms offer a venue for patients and physicians to exchange health information and provide health care services outside traditional organizational contexts. Previous studies have seldom focused on the factors that drive the proactivity of physicians’ online behavior. Additionally, there is limited research exploring the influence of patients on physicians’ online behavior, particularly from the perspective of patients possessing power. Objective: This study aims to investigate the effect of patient-empowering behaviors on physicians’ online knowledge sharing and uncover the potential mechanisms of proactivity. Based on the proactive motivational model and empowerment theory, we propose the existence of a reverse empowerment process, where empowerment flows from patients to physicians. We suggest that patient-empowering behaviors may drive physicians’ online knowledge sharing as a form of proactivity. Specifically, 3 proactive motivational states—knowledge-sharing self-efficacy, sharing meaning, and positive professional affect—mediate this relationship. Additionally, platform extrinsic rewards, as a contextual factor, have a moderating effect. Methods: To validate our proposed research model, we conducted a survey in China using the WJX platform, targeting physicians engaged in online knowledge sharing. The measurement instrument utilizes validated items adapted from prior research, using a 5-point Likert scale. We collected 257 valid responses, ensuring that participation was both anonymous and voluntary. Data analysis was performed in 2 stages. The first stage assessed the measurement model for reliability and validity, using the Harman 1-factor test and confirmatory factor analysis. The second stage used partial least squares-structural equation modeling to examine the direct, moderation, and mediation effects among the constructs, with bootstrapping used for significance testing. This comprehensive approach ensures a robust analysis of the proposed hypotheses and contributes to the overall validity of our research model. Results: Perceived patient-empowering behaviors significantly and positively influence physicians’ online knowledge sharing (β=0.27, P<.001). Knowledge-sharing self-efficacy (effect=0.06, P=.04), sharing meaning (effect=0.12, P<.001), and positive professional affect (effect=0.10, P=.003) each partially mediate the effect of patient-empowering behaviors on physicians’ online knowledge sharing. The overall proactive motivational states play a complete mediation role, meaning the entire indirect effect of the model is significant (effect=0.29, P<.001), while the direct effect in the model is nonsignificant (effect=0.07, P=.26). Additionally, platform extrinsic rewards significantly and negatively moderate the effect of sharing meaning on physicians’ online knowledge sharing (β=–0.13, P=.001). Conclusions: This study is the first to recognize and examine proactivity as an alternative mediating mechanism for physicians’ online knowledge sharing, highlighting the active role of patients in empowering physicians. It makes a significant contribution to the existing literature on empowerment, eHealth, and proactive behavior. Additionally, the findings offer valuable guidance for designing and managing digital platforms to ensure service sustainability.