We investigated the role of Lgi3-4 proteins in cardiac electrophysiology, with a specific focus on IKur, and their potential contribution to the pathophysiology of atrial fibrillation (AF). In human atrial tissue and heterologous cells, Lgi3 and Lgi4 interacted with KV1.5 channels. In a mouse model of AAV9-mediated cardiac-specific Lgi4 expression, sinoatrial and atrioventricular conduction were impaired, resulting in a prolonged QRS interval. Action potential repolarisation was delayed in atrial and ventricular cardiomyocytes from Lgi4 mice. In HEK293 cells, Lgi3-4 impaired KV1.5/KVβ association, partially reversing the KVβ-induced inactivation and reducing IKur amplitude. These results correlated with the reduced KV1.5 membrane expression and IKur density observed in Lgi4 cardiomyocytes and HEK293 cells. Notably, Lgi4 protein expression was lower in atrial tissue from patients with AF than in sinus rhythm patients. The reduced Lgi4 protein levels in AF were also associated with an altered colocalisation with KV1.5 channels, suggesting potential disruption in their functional interactions. We conclude that Lgi3-4 proteins are new components of the cardiac KV1.5 channelosome. They modulate IKur by interfering with the KV1.5-KVβ interaction. Significantly, Lgi4 is dysregulated differently in paroxysmal vs. permanent AF. The results identify Lgi3-4 proteins as novel regulators of the KV1.5 channelosome, opening new pathways for investigating the role of IKur dysfunction in the mechanism of atrial fibrillation.