Aims: To describe and compare family functioning, family health, and perceived social support from nurses and to identify the variables that are associated with family functioning in patients with heart failure (HF) and their family members in Denmark, Iran, and Iceland. Design: An international multi-centre cross-sectional study. Methods: A sample of 1382 participants (692 patients and 690 family members) from Denmark, Iceland, and Iran were included from January 2015 to May 2020. Data were collected using the Family Functioning, Health, and Social Support questionnaire. Results: The significant factors associated with family functioning in patients were country, New York Heart Association classification (NYHA), education level, age, family health, social support, and there was a significant interaction effect between NYHA class and gender. The significant factors associated with family functioning in family members were country, education level, work status, family health, and there was a significant interaction effect between education and work status. Conclusion: This study indicated that the strongest factor associated with higher family functioning was family health for both patients and family members. Women in NYHA class I and younger patients and those with an academic education had a lower level of family functioning. Moreover, unemployed family members with an elementary education and family members with elementary and high school educations who were self-employed or employees had a lower level of family functioning. Impact: This is the first international study to investigate family functioning, family health, and social support and adds to the literature on the factors associated with family functioning in patients with HF and their family members. Our findings may help nurses to identify the most vulnerable families living with HF, thereby being able to provide special support to enhance their family functioning to promote self-management strategies.