Catecholaminergic polymorphic ventricular tachycardia is a form of exercise-induced sudden cardiac death that has been linked to mutations in the cardiac Ca²⁺ release channel/ryanodine receptor (RyR2) located on the sarcoplasmic reticulum (SR). We have shown that catecholaminergic polymorphic ventricular tachycardia-linked RyR2 mutations significantly decrease the binding affinity for calstabin-2 (FKBP12.6), a subunit that stabilizes the closed state of the channel. We have proposed that RyR2-mediated diastolic SR Ca²⁺ leak triggers ventricular tachycardia (VT) and sudden cardiac death. In calstabin-2-deficient mice, we have now documented diastolic SR Ca²⁺ leak, monophasic action potential alternans, and bidirectional VT. Calstabin-deficient cardiomyocytes exhibited SR Ca²⁺ leak-induced aberrant transient inward currents in diastole consistent with delayed after-depolarizations. The 1,4-benzothiazepine JTV519, which increases the binding affinity of calstabin-2 for RyR2, inhibited the diastolic SR Ca²⁺ leak, monophasic action potential alternans and triggered arrhythmias. Our data suggest that calstabin-2 deficiency is as a critical mediator of triggers that initiate cardiac arrhythmias.