Among >300 miRNAs known to date, miR-1 is considered muscle-specific. Here we show that that miR-1 overexpressed in individuals with coronary artery disease, and when overexpressed, it exacerbated arrhythmogenesis in both infarcted and normal hearts of rats whereas elimination of miR-1 by its antisense inhibitor relieved it. MiR-1 rendered slowed conduction and depolarized membrane by post-transcriptionally repressing KCNJ2 and GJA1 genes, likely accounting for its arrhythmogenic potential. Thus, miR-1 may have important pathophysiological functions in heart, being a novel antiarrhythmic target useful in the treatment and prevention of various cardiac pathologies.