AIMS: Titin truncating variants (TTNtv) are a major genetic cause of dilated cardiomyopathy (DCM), accounting for approximately 25% of familial cases. Atrial fibrillation (AF) frequently occurs in DCM patients carrying TTNtv and may precede overt ventricular dysfunction, suggesting an atrial-specific disease mechanism. How TTNtv increase susceptibility to AF, particularly in the absence of established DCM, remains incompletely understood. This study aimed to define the cellular and molecular mechanisms by which a clinically relevant TTNtv predisposes to atrial arrhythmogenesis. METHODS AND RESULTS: We introduced a patient-associated TTNtv (TTN c.59926+1G>A) into human induced pluripotent stem cell-derived atrial cardiomyocytes (hiPSC-CMs). TTNtv hiPSC-CMs exhibited proarrhythmic electrophysiological alterations, including increased spontaneous beating frequency, abnormal sodium channel kinetics, and heightened sensitivity to cholinergic agonists. In silico simulations demonstrated that heightened cholinergic sensitivity was sufficient to trigger AF under conditions of sinus tachycardia. RNA sequencing revealed dysregulation of sarcomere assembly and extracellular matrix pathways, and TTNtv hiPSC-CMs showed structurally shortened sarcomeres. Engineered heart tissues composed of TTNtv hiPSC-CMs co-cultured with cardiac fibroblasts demonstrated reduced contractile force and increased secretion of collagen, fibronectin-1 and TGF-β1, consistent with activation of profibrotic signalling. Together, these findings indicate that a TTNtv can cause intrinsic atrial electrical instability and promote pro-fibrotic signalling. CONCLUSION: Our results identify atrial electrophysiological abnormalities and profibrotic remodelling as key mechanisms by which TTNtv increase AF risk, even in the absence of overt DCM. These findings support a primary atrial contribution to TTNtv-associated arrhythmogenesis and provide mechanistic insight into AF as an early clinical manifestation in carriers.