Background Negative emotions are linked to the onset and development of coronary heart diseases (CHD), whereas positive emotions are associated with better health and lower mortality rates among patients with these diseases. The objective of this randomised trial was to improve cardiac patients' emotional states using a Programme to Improve Well‐being (PIW) based exclusively on positive interventions (those that promote intentional behaviours and thoughts to improve well‐being). Methods Cardiac patients (n = 108) were randomly assigned to two parallel groups. In the control group, they participated in only a Cardiac Rehabilitation Programme (CRP group), whereas the intervention group also participated in the PIW (CRP+PIW group). Physical functional capacity, depressive symptoms, hostility, and negative and positive affect were assessed at T1 (baseline) and T2 (8 weeks later). Results At T2, after controlling for functional capacity, the CRP+PIW group reported a significantly less negative affect than the CRP group. Moreover, the CRP group did not change from T1 to T2, whereas the CRP+PIW group reported more positive emotions and fewer negative emotions and hostility at T2 than at T1. Conclusions Positive interventions effectively improve the emotional state of cardiac patients. We suggest that specific modules should be included in the CRP to improve well‐being.