In Japan, a considerable number of foreigners encounter challenges in accessing appropriate healthcare services due to the lack of insurance coverage. However, the absence of a public database on these individuals makes it difficult to assess their health problems and healthcare access status. This study aims to investigate the characteristics of vulnerable Thai patients in Japan and to shed light on the specific challenges they face within Japan’s healthcare system. A retrospective analysis was conducted using records of patients who required emergency healthcare support from the Royal Thai Embassy in Tokyo between 2004 and 2020. Descriptive statistical analyses were performed to examine the general characteristics, insurance status, and diseases of the patients. Additionally, patients were classified as either prolonged residents or brief residents based on their duration of stay in Japan until hospital admission (1 year or more or less than 1 year). A total of 74 patients were identified, with the majority (91.9%) lacking insurance coverage. Notably, there was an increase in the number of brief residents, including tourists, during the 2010s. Prolonged residents were more likely to experience chronic diseases, whereas brief residents were more prone to sustaining injuries. The patient records from the Thai Embassy consistently highlight the urgent requirement for emergency healthcare support within this population. However, the existing policies in Japan fall short in adequately addressing the healthcare access needs of this vulnerable population. Therefore, it is crucial to provide additional support and interventions to enhance their healthcare access.