Rationale: Athletes present with atrioventricular node (AV node) dysfunction manifesting as AV block. This can necessitate electronic pacemaker implantation, known to be more frequent in athletes with a long training history. Objective: AV block in athletes is attributed to high vagal tone. Here we investigated the alternative hypothesis that electrical remodeling of the AV node is responsible. Methods and Results: Radio-telemetry ECG data and AV node biopsies were collected in sedentary and trained Standardbred racehorses, a large-animal model of the athlete's heart. Trained horses presented with longer PR intervals (that persisted under complete autonomic block) versus sedentary horses, concomitant with reduced expression of key ion channels involved in AV node conduction: L-type Ca2+ channel subunit CaV1.2 and the hyperpolarization-activated cyclic nucleotide gated channel 4 (HCN4). AV node electrophysiology was explored further in mice; prolongation of the PR interval (in vivo and ex vivo), Wenckebach cycle length and AV node refractory period was observed in mice trained by swimming versus sedentary mice. Transcriptional profiling in laser-capture microdissected AV node revealed striking reduction in pacemaking ion channels in trained mice, translating into protein downregulation of CaV1.2 and HCN4. Correspondingly, patch clamp recordings in isolated AV node myocytes demonstrated a training-induced reduction in ICa,L and If density that likely contributed to the observed lower frequency of action potential firing in trained cohorts. microRNA (miR) profiling and in vitro studies revealed miR-211-5p and miR-432 as direct regulators of CaV1.2 and HCN4. In vivo miRs suppression or detraining restored training-induced PR prolongation and ion channel remodeling. Conclusions: Training-induced AV node dysfunction is underscored by likely miR-mediated transcriptional remodeling that translates into reduced current density of key ionic currents involved in impulse generation and conduction. We conclude that electrical remodeling is a key mechanism underlying AV block in athletes.