Heart failure with preserved ejection fraction (HFpEF) is frequently complicated by atrial fibrillation (AF), yet the mechanisms underlying AF vulnerability prior to or independent of extensive structural remodeling remain incompletely understood. In this study, we investigated atrial electrophysiological remodeling in a murine HFpEF model induced by combined high-fat diet and L-NAME administration. The model reproduced key HFpEF features, including obesity, hypertension, preserved systolic function, diastolic dysfunction, and impaired exercise capacity. Transesophageal atrial pacing revealed a marked increase in AF inducibility in HFpEF mice. Ex vivo optical mapping demonstrated prolonged action potential duration, global conduction slowing, and increased conduction heterogeneity in the left atrium. Notably, atrial fibrosis, inflammation, and atrial enlargement were absent. Instead, both gene and protein expression analyses showed a selective downregulation of connexin 40 (Cx40), while connexin 43 expression was preserved. Immunohistochemistry confirmed reduced expression and altered localization of Cx40 in atrial myocardium. These findings indicate that Cx40 downregulation and gap junction dysfunction precede structural remodeling and constitute a primary electrophysiological AF substrate. Our results highlight impaired atrial conduction as a primary mechanism of AF vulnerability in this metabolic HFpEF model and provide new insights into the pathogenesis of HFpEF-associated AF that may inform future preventive strategies.