Background: Support from the family positively affects self-management, patient outcomes and the incidence of hospitalizations among patients with heart failure (HF). To involve family members in HF care is thus valuable for the patients. Registered nurses (RNs) frequently meet family members to patients with HF and the quality of these encounters are likely to be influenced by the attitudes RNs hold toward families. Aims: To explore RNs' attitudes toward the importance of families' involvement in HF nursing care and to identify factors that predict the most supportive attitudes. Methods: Cross-sectional, multicentre web-survey study. A sample of 303 RNs from 47 hospitals and 30 primary health care centres (PHCC) completed the instrument Families’ Importance in Nursing Care - Nurses’ Attitudes. Results: Overall, RNs were supportive of families' involvement. Nonetheless, attitudes toward inviting families to actively take part in HF nursing care and involve families in planning of care were less supportive. Factors predicting the most supportive attitudes were to work in a PHCC, a HF clinic, a workplace with a general approach toward families, to have a postgraduate specialization, education in cardiac and or HF nursing care, and a competence to work with families. Conclusions: Experienced RNs in HF nursing care can be encouraged to mentor their younger and less experienced colleagues to strengthen their supportive attitudes toward families. RNs who have designated consultation time with patients and families, as in a nurse-led HF clinic, may have the most favourable condition for implementing a more supportive approach to families.