Aim: This study was conducted to investigate the relationships between caregiving stress, mental health and physical health in family caregivers of adult patients with cancer at a University Teaching Hospital in Jeddah city, Saudi Arabia.; Methods: A cross-sectional correlational study was carried out with a convenience sample of 160 family caregivers of adult patients with cancer. Data were collected using a self-administered questionnaire including the Modified Caregiver Strain Index, the DUKE Health Profile and sociodemographic items. The data were analysed using the Statistical Package for the Social Sciences (SPSS). Descriptive and inferential statistics and correlations were performed.; Results: Participants experienced a certain level of caregiving stress (M = 9.01, SD = 5.645). Many factors were found to be correlated to higher caregiving stress in this study. Caregiving stress showed significant moderate negative correlations with mental and physical health (p < 0.01). Statistically significant differences were found between age, gender, nationality, education, monthly income, and caregiving stress or DUKE Health Profile scores (p < 0.05).; Conclusions: Caregiving stress affects family caregivers' mental and physical health. Such stress can disrupt the caregiving performance of family caregivers. Discovering the causes of caregiving stress among the family caregivers of adult patients with cancer may help to determine the main elements affecting patient care and can assist oncology nurses in providing support and services to caregivers. Educational strategies/intervention programs in the hospitals may be required to reduce caregiving stress levels and improve the health and well-being of family caregivers of adult patients with cancer.