Atrial fibrillation (AF)-related remodelling may persist during sinus rhythm (SR) and manifest as subtle P-wave abnormalities. This study investigated whether P-wave phenotypes from SR electrocardiograms can distinguish veteran endurance athletes withprevious AF from athletes without AF history. Twenty-three male athletes with prior AF (AF-SR; 56.30 ± 5.63 years) and 20 male controls (SR-SR; 56.55 ± 5.25 years) underwent 10-min resting 12-lead ECG at 180 Hz. A representative 10-s segment was analysed per participant; 328 P-waves were manually annotated using V3 as the reference lead, and patient-level phenotypes were derived. AF-SR athletes showed greater P-wave area variability (5.94 [5.24-6.34] vs 4.14 [3.80-4.73] mV·ms, p < 0.0001), energy variability (0.20 [0.14-0.22] vs 0.10 [0.085-0.13] mV², p = 0.001), amplitude variability (0.035 [0.030-0.038] vs 0.024 [0.022-0.026] mV, p = 0.001), and biphasic index (0.13 [0.10-0.15] vs 0.09 [0.06-0.10], p = 0.002). Random forest achieved an AUC of 0.837 versus 0.815 for logistic regression. These findings support sinus-rhythm P-wave phenotyping as a non-invasive marker of latent AF-related atrial substrate in veteran endurance athletes.