Background: The internet has greatly improved the availability of medical knowledge and may be an important avenue to improve patients’ trust in physicians and promote primary healthcare seeking by reducing information asymmetry. However, very few studies have addressed the interactive impacts of both patients’ internet use and trust on primary healthcare-seeking decisions. Objective: To explore the impact of internet use on the relationship between patients’ trust in physicians and primary healthcare seeking among Chinese adults 18 years of age and older to understand the varieties of effects in different cities. Methods: Generalized linear mixed models were applied to investigate the interactive impacts of internet use and patients’ trust in physicians on primary healthcare seeking using pooled data from the China Family Panel Study of 2014 to 2018. We also compared these effects based on different levels of urbanization, ages, and PHC services. Results: Overall, a higher degree of patients’ trust (p < 0.001) was able to directly predict better primary healthcare seeking, and internet use significantly increased the positive effect of patients’ trust on primary healthcare seeking (p < 0.001). However, the marginal effect analysis showed that this effect was related to the level of patients’ trust and that internet use could reduce the positive effect of patients’ trust on primary healthcare seeking when the individual had a low level of trust (≤ 3 units). Further, the heterogeneity analysis indicated that the benefits from internet use were higher in cities with high urbanization, high aging, and high PHC service levels compared to cities with low levels of these factors. Conclusions: The internet use may enhance patients’ trust-related PHC utilization. However, this impact is effective only if patients’ benchmark trust remains at a relatively high level. Comparatively, the role of internet use is more effective in areas with high urbanization, high aging and high PHC level. Thus, with increasing accessibility to the internet, the internet should be regulated to disseminate correct healthcare information. Moreover, in-depth integration of the internet and PHC should be promoted to provide excellent opportunities for patient participation, and different strategies should be set according to each city’s characteristics.