A large number of studies have linked diabetes mellitus (DM) to the development of cardiovascular disease. However, the association between DM and arrhythmic disease of the heart is less well recognized by the cardiology community. Mostly, the focus is the increased thromboembolic risk that the presence of DM confers to patients with atrial fibrillation (AF). However, recent evidence points toward an increased prevalence of AF in patients with diabetes. This association seems to be reinforced by the presence of additional comorbidities such as hypertension and insulin-resistance. Moreover, different pathophysiological mechanisms support this association. Notably the link between AF and DM is bidirectional: not only patients with DM are more vulnerable to develop AF but also patients with new-onset AF have an increased risk of DM. Herein, the epidemiological findings and pathophysiological mechanisms linking AF and DM will be reviewed in order to increase the knowledge on this association and improve the clinical and therapeutic approach.