The most prevalent form of cardiac rhythm abnormality among older populations is atrial fibrillation (AF). The prognostic nutritional index (PNI) is a reliable predictor of mortality in various diseases. The association between the PNI and mortality among AF patients over 80 years remains uncleared.A retrospective assessment of AF cases admitted to a single cardiovascular disease unit in China between January 2015 and June 2020 was performed. The PNI at admission was defined as 10 × serum albumin (g/dL) + 0.005 × total lymphocyte count (per mm3). The association between PNI and cardiovascular disease (CVD)-related or all-cause mortality within 28 days was assessed via multivariable Cox regression. The analysis included 1141 patients. The CVD-related and all-cause mortality rates were 3.3% and 8.7%. Kaplan-Meier analyses revealed that cases with lower PNI tertiles exhibited higher all-cause mortality (T1: 7.6%; T2: 6.1%; T3: 2.4%, P < 0.001) or CVD mortality (T1: 6.3%; T2: 2.9%; T3: 0.8%, P < 0.001). After adjusting for potential confounders, continuous PNI was negatively related to the hazard of all-cause mortality (HR 0.92, 95% CI 0.89, 0.96) and CVD-related mortality (HR 0.90, 95% CI 0.84, 0.95). Compared to the T1 group, patients with a higher PNI exhibited a lower risk of all-cause mortality (P for trend 0.003) and CVD-related mortality (P for trend 0.005). Among most subgroups, CVD-related and all-cause mortality decreased with elevating PNI values.PNI is significantly negatively correlated with CVD-related and all-cause mortality among AF cases over 80 years.