Due to liberalized immigration policies in recent years, the number of foreign residents in Japan has surged. However, the issue of some foreign residents lacking public health insurance persists. Given that the entities responsible for premium contributions to National Health Insurance (NHI) essentially make it a voluntary system, this study is the first to utilize nationwide microdata to investigate the factors preventing foreign residents from enrolling in NHI. This study offers several new insights. Specifically, in businesses where enrollment in Employees’ Health Insurance is not mandatory, foreign residents working as regular full-time employees are significantly less likely to enroll in NHI. Additionally, individuals who are job-seeking or unemployed, those engaged in other forms of employment—including family workers and trainees—and those with no health concerns are all significantly less likely to enroll.In the analysis restricted to individuals other than regular full- time employees, the economic conditions of the prefecture of residence and whether the individual’s country of nationality has implemented universal health insurance significantly affect enrollment. Furthermore, our analysis suggests that, while the large number of new positive cases following the prolonged spread of the pandemic played a role, the initial panic caused by the outbreak may have served as a significant incentive for foreign residents in Japan to enroll in NHI.