Aims Caregiver characteristics and behaviours are associated with patients' and caregivers' outcomes. However, there is scarce evidence on interventions aimed at improving caregiver contribution (CC) to patient self-care, caregiver self-efficacy, and caregiver preparedness in heart failure (HF). This study aims to evaluate the effect of Motivational Interviewing (MI) on CC to patient self-care, caregiver self-efficacy, and caregiver preparedness in HF. Methods and results This is a secondary outcome analysis of MOTIVATE-HF randomized controlled trial. Caregivers of patients with HF were randomized into three arms: Arm 1 (MI for patients), Arm 2 (MI for patients and caregivers), and Arm 3 (standard care). The intervention consisted of one MI session plus three telephone contacts. Data were collected at baseline and after 3, 6, 9, and 12 months. A total of 510 patient and caregiver (median age 54 years, 76% females) dyads were enrolled, 235 caregivers and 238 patients completed the study until 12 months. Caregiver contribution to self-care maintenance and management and caregiver preparedness significantly improved over time, but without significant differences among the three arms. At 9 months, caregiver self-efficacy improved more in Arm 2 than Arm 3 [difference: 8.36, 95% confidence interval (CI) (3.13; 13.59), P = 0.001]. This improvement remained significant also at linear mixed model [ β ^ = 1.39, 95% CI (0.02; 2.75), P = 0.046]. Conclusion Motivational Interviewing did not improve CC to patient self-care and caregiver preparedness. However, it increased caregiver self-efficacy, which is known to be a powerful mediator of CC to self-care. Registration Clinicaltrials.gov: NCT02894502. [ABSTRACT FROM AUTHOR]