Background: Patients and clinicians report that fatigue post-myocardial infraction (MI) is a bothersome symptom during recovery. Aim: The objective of this study was to explore whether there is a relationship between fatigue, cardiac self-efficacy, stress, breathlessness and physical activity one year post-MI. Method: Data were collected from a sample of patients diagnosed with MI one year earlier (n = 125) who responded to a questionnaire package measuring fatigue, cardiac self-efficacy, physical activity and the symptoms breathlessness and stress. Correlation and regression analyses were preformed to evaluate which factors were related to fatigue. Results: The results showed that cardiac self-efficacy was associated with fatigue (r = −0.611, p = 0.01) and the regression model, controlling for breathlessness and stress, showed an explained variance of 72% one year post-MI. Physical activity was not significant in this model and did not predict fatigue during this time period. Conclusion: Post-MI fatigue-relief support should rely not only on identification of fatigue and other concurrent symptoms, but also on identification of cardiac self-efficacy.