BackgroundMultimorbidity management can be extremely challenging in patients with dementia. This study aimed to elucidate the approaches of primary care physicians in Japan and the United States (US) in managing multimorbidity for patients with dementia and discuss the challenges involved.MethodsThis qualitative study was conducted through one-on-one semi-structured interviews among primary care physicians, 24 each from Japan and Michigan, US. Thematic and content analyses were performed to explore similarities and differences among each country’s data.ResultsPrimary care physicians in Japan and Michigan applied a relaxed adherence to the guidelines for patients’ chronic conditions. Common challenges were the suboptimal consultation time, the insufficient number or ability of care-coordinating professionals, patients’ conditions such as difficulties with self-management, living alone, behavioral issues, and refusal of care support. Unique challenges in Japan were free-access medical systems and not being sure about the patients’ will in end-of-life care. In Michigan, physicians faced challenges in distance and lack of transportation between clinics and patients’ homes and in cases where patients lacked the financial ability to acquire good care.ConclusionsTo improve the quality of care for patients with multimorbidity and dementia, physicians would benefit from optimal time and compensation allocated for this patient group, guidelines for chronic conditions to include information regarding changing priority for older adults with dementia, and the close collaboration of medical and social care and community resources with support of skilled care-coordinating professionals.Supplementary InformationThe online version contains supplementary material available at 10.1186/s12875-023-02088-4.