The development of online health platforms in recent years has drawn significant research attention to understanding patient participation. However, the unfairness in patient–doctor relationship development has been largely overlooked in the online health context. This study proposes and tests a model that examines how interactional unfairness (encompassing interpersonal unfairness and informational unfairness) influences online patient–doctor relationship quality and the contingent conditions of a doctor’s professional seniority and disease severity on the unfairness–relationship quality link. Using objective data with 31,521 observations from a leading online health platform, this study employed rare-event logistic regression to test the model. The results show that interpersonal unfairness and informational unfairness have negative and positive effects on relationship quality, respectively, and a doctor’s professional seniority and disease severity moderate the strength of the unfairness-relationship quality link. This study advances the knowledge of interactional unfairness in online health, and provides practical insights for online healthcare stakeholders into how to manage unfairness and consider the contingent factors to improve patient–doctor relationship in online health consultations.