Heart muscle excitation–contraction (E-C) coupling is governed by Ca²⁺ release units (CRUs) whereby Ca²⁺ influx via L-type Ca²⁺ channels (Cav1.2) triggers Ca²⁺ release from juxtaposed Ca²⁺ release channels (RyR2) located in junctional sarcoplasmic reticulum (jSR). Although studies suggest that the jSR protein triadin anchors cardiac calsequestrin (Casq2) to RyR2, its contribution to E-C coupling remains unclear. Here, we identify the role of triadin using mice with ablation of the Trdn gene (Trdn^−/−). The structure and protein composition of the cardiac CRU is significantly altered in Trdn^−/− hearts. jSR proteins (RyR2, Casq2, junctin, and junctophilin 1 and 2) are significantly reduced in Trdn^−/− hearts, whereas Cav1.2 and SERCA2a remain unchanged. Electron microscopy shows fragmentation and an overall 50% reduction in the contacts between jSR and T-tubules. Immunolabeling experiments show reduced colocalization of Cav1.2 with RyR2 and substantial Casq2 labeling outside of the jSR in Trdn^−/− myocytes. CRU function is impaired in Trdn^−/− myocytes, with reduced SR Ca²⁺ release and impaired negative feedback of SR Ca²⁺ release on Cav1.2 Ca²⁺ currents (I_Ca). Uninhibited Ca²⁺ influx via I_Ca likely contributes to Ca²⁺ overload and results in spontaneous SR Ca²⁺ releases upon β-adrenergic receptor stimulation with isoproterenol in Trdn^−/− myocytes, and ventricular arrhythmias in Trdn^−/− mice. We conclude that triadin is critically important for maintaining the structural and functional integrity of the cardiac CRU; triadin loss and the resulting alterations in CRU structure and protein composition impairs E-C coupling and renders hearts susceptible to ventricular arrhythmias.