BACKGROUND Cardiomyopathy associated with thymoma is thought to be a cardiac manifestations of myasthenia gravis (MG). However, there are case reports of newly diagnosed thymoma presenting with cardiomyopathy without MG, and the mechanism remains unclear. The purpose of this report is to explore tachycardia-induced cardiomyopathy (TIC) as a potential mechanism for cardiomyopathy in thymoma without features of MG. CASE REPORT A 31-year-old man presented with atrial flutter with right bundle branch block and severe biventricular heart failure. Echocardiogram revealed severe left ventricle ejection fraction (LVEF) of 15% with biventricular dilation with impaired systolic function. Computer tomography coronary angiography demonstrated normal coronary artery disease. Cardiac magnetic resonance imaging showed normal T1 and T2 mapping, without inflammation or edema. A large anterior mediastinal mass was found on computer tomography chest. Mediastinal mass biopsy identified type B3 thymoma (WHO classification) with dual population of large, uniform epithelial thymic cells and immature T cell phenotype. Acetylcholine receptor antibody was positive without clinical features of MG and hypogammaglobulinemia indicating Good syndrome. He was treated with antiarrhythmic and heart failure pharmacotherapy, carboplatin and paclitaxel, and intravenous immunoglobulin. He demonstrated reversible heart failure following abolishment of tachyarrhythmia, consistent with tachycardia-induced cardiomyopathy. CONCLUSIONS We report a rare case of a newly diagnosed thymoma and Good syndrome without clinical features of MG presenting with tachyarrhythmia and severe biventricular failure. The reversibility of the cardiomyopathy following abortion of tachyarrhythmia with treatment highlights TIC as a potential cause.