Tachycardia-induced cardiomyopathy (TICM) is typically reversible with rhythm control, but individual susceptibility remains poorly understood and may reflect genetic predisposition.A 66-year-old woman with paroxysmal atrial fibrillation (AF) presented with new-onset heart failure. Genetic testing identified a likely pathogenic heterozygous ABCC9 gene variant (c.3892+2T>C), not previously associated with dilated cardiomyopathy or AF. ABCC9 loss-of-function mutations have been linked with cardiac channelopathies and cardiomyopathies. Ventricular function improved with rhythm control and medical therapy.This case illustrates the potential role of ABCC9 mutations in arrhythmia-induced cardiomyopathy beyond pure TICM. Although this variant has not been previously reported in affected individuals, existing models support its pathogenicity. The co-occurrence of prolonged QT, familial AF, and dilated cardiomyopathy underscores the value of genetic testing in cardiac disease.Genetic testing may reveal causes in atypical or treatment-resistant cardiomyopathies and arrhythmias. This novel ABCC9 variant suggests a genetic contribution to AF-induced cardiomyopathy beyond the expected course of TICM.