Abstract Aims Patients with heart failure exhibit poor adherence to self‐care behaviours. Recent studies have shown a link between Type D personality, social support, and self‐care behaviours of patients with chronic diseases. However, the mechanisms underlying this relationship have received little empirical attention in those with heart failure. This study aimed to identify the mediating effects of social support in the relationship between Type D personality and self‐care behaviours in patients with heart failure. Design This study employed a descriptive cross‐sectional design. Methods The sample comprised 170 patients aged over 18 years who had been diagnosed with heart failure for over 6 months and visited a tertiary care university hospital in Seoul. From April–July 2019, data were collected through an interview conducted using a structured questionnaire and a review of medical records by two trained research assistants. Multiple regression models were tested using the SPSS PROCESS procedure with bootstrapping to determine the significance of the mediation. Results The prevalence of Type D personality in patients with heart failure was 20.0%. Significant associations were identified among Type D personality, social support, and self‐care behaviours. Results revealed an indirect effect, suggesting that the effects of Type D personality on self‐care behaviours were mediated by social support. Conclusion Heart failure patients with Type D personality were likely to lack social support, which may have influenced their low adherence to self‐care behaviours. Thus, to facilitate long‐lasting self‐care behaviours, it is vital to provide supportive care or educational programs for patients with Type D personality. Impact Providing sustainable social support can have a positive impact on the long‐term self‐care maintenance of patients with heart failure patients who have a Type D personality. Future studies are required to further elucidate the underlying mechanisms of the relationship between Type D personality, social support, and self‐care behaviours.