Background:Atrial fibrillation (AF) begets AF in part due to atrial remodeling, the molecular mechanisms of which have not been completely elucidated. This study was conducted to identify microRNA(s) responsible for electrical remodeling in AF.Methods and Results:The expression profiles of 1205 microRNAs, in cardiomyocytes from patients with persistent AF and from age-, gender-, and cardiac function-matched control patients with normal sinus rhythm, were examined by use of a microRNA microarray platform. Thirty-nine microRNAs differentially expressed in AF patients’ atria were identified, including miR-30d, as a candidate responsible for ion channel remodeling by in silico analysis. MiR-30d was significantly upregulated in cardiomyocytes from AF patients, whereas the mRNA and protein levels ofCACNA1C/Cav1.2 andKCNJ3/Kir3.1, postulated targets of miR-30d, were markedly reduced.KCNJ3/Kir3.1 expression was downregulated by transfection of the miR-30 precursor, concomitant with a reduction of the acetylcholine-sensitive inward-rectifier K⁺current (I_K.ACh).KCNJ3/Kir3.1 (but notCACNA1C/Cav1.2) expression was enhanced by the knockdown of miR-30d. The Ca²⁺ionophore, A23187, induced a dose-dependent upregulation of miR-30d, followed by the suppression ofKCNJ3mRNA expression. Blockade of protein kinase C signaling blunted the [Ca²⁺]_i-dependent downregulation of Kir3.1 via miR-30d.Conclusions:The downward remodeling ofI_K.AChis attributed, at least in part, to deranged Ca²⁺handling, leading to the upregulation of miR-30d in human AF, revealing a novel post-transcriptional regulation ofI_K.ACh. (Circ J 2016; 80: 1346–1355)