Cancer of unknown primary site (CUP) is an aggressive disease with poor prognosis. As research on the experiences of CUP patients and their families is scarce, this study aimed to compare the family caregiver-perceived experiences of CUP patients and their families with those of patients with known primary cancer. The study used a nationwide survey to collect data from family caregivers of CUP patients and patients with known primary cancer. The results showed that family caregivers of CUP patients experienced higher levels of burden, anxiety, and depression compared to family caregivers of patients with known primary cancer. The study highlights the need for more research on the experiences of CUP patients and their families and the development of targeted interventions to support them.