Background: Internet hospitals show great potential for adequately fulfilling people's demands for high-quality outpatient services, and with the normalization of the epidemic prevention and control of COVID-19, internet hospitals play an increasingly important role in delivering health services to the public. However, the factors that influence patients' intention to use the online inquiry services provided by internet hospitals remain unclear. Understanding the patients' behavioral intention is necessary to support the development of internet hospitals in China and promote patients' intention to use online inquiry services provided by internet hospitals during the prevention and control of the COVID-19 epidemic. Objective: The purpose of this study is to identify the determinants of patients' intention to use the online inquiry services provided by internet hospitals based on the theory of planned behavior (TPB). Methods: The hypotheses of our research model were developed based on the TPB. A questionnaire was developed through patient interviews, verified using a presurvey, and used for data collection for this study. The cluster sampling technique was used to include respondents with chronic diseases. Structural equation modeling was used to test the research hypotheses. Results: A total of 638 valid responses were received from patients with chronic diseases. The goodness-of-fit indexes corroborated that the research model was a good fit for the collected data. The model explained 45.9% of the variance in attitude toward the behavior and 60.5% of the variance in behavioral intention. Perceived behavioral control and perceived severity of disease had the strongest total effects on behavioral intention (beta=.624, P=.004 and beta=.544, P=.003, respectively). Moreover, perceived convenience, perceived information risk, emotional preference, and health consciousness had indirect effects on behavioral intention, and these effects were mediated by attitude toward the behavior. Among the four constructs, perceived convenience had the highest indirect effect on behavioral intention (beta=.207; P=.001). Conclusions: Perceived behavioral control and perceived severity of disease are the most important determinants of patients' intention to use the online inquiry services provided by internet hospitals. Therefore, internet hospitals should further optimize the design of online service delivery and ensure a reasonable assembly of high-quality experts, which will benefit the promotion of patients' adoption intention toward online inquiry services for health purposes. Perceived convenience, emotional preference, and perceived risks also have effects on behavioral intention. Therefore, the relevant quality control standards and regulations for internet hospitals should be further developed and improved, and the measures to protect personal information should be strengthened to ensure the patient safety. Our study supports the use of the TPB in explaining patients' intention to use online inquiry services provided by internet hospitals.