Nutritional problem of the patient is a valuable stiuation in patients undergoing surgery. In this study, we aimed to investigate the relationship between prognostic nutritional index (PNI) and postoperative in-mortality in patients undergoing cardiac surgery. 1003 patients undergoing on-pump cardiac surgery in our hospital between January 2016–January 2020 were included in this study retrospectively. Patients were divided into two groups based on in-hospital mortality, as survivors (Group I, n = 934) and non-survivors (Group II, n = 69). Their preoperative nutritional status was determined using the PNI. Compared to survivors, non-survivors were found to have a significantly higher mean age (62.5 \pm 10.8 vs. 67.45 \pm 10.1, P 0.001) and significantly lower mean preoperative ejection fraction (51.6 \pm 0.3 vs. 44.5 \pm 1.2, P 0.001). And combined cardiac surgery rate was significantly higher in non-survivors (P = 0.009). Also non-survivors had a significantly lower mean PNI compared to survivors (44.76 \pm 7.63 vs. 48.34 \pm 6.71, P 0.001). Multivariate analysis Model 1 revealed that age (Odds ratio (OR): 1.756; 95% confidence interval (CI): 1.250–3.790, P = 0.029), intra-aortic balloon pump usage (OR: 2.252, 95% CI: 1.885–6.194, P 0.001), combined cardiac surgery (OR: 0.542, 95% CI: 0.428–0.690, P = 0.041) and the PNI (OR: 0.639, 95% CI: 0.552–0.874, P = 0.021) were independent predictors of mortality. In Model 2, age >70 (OR: 2.437, 95% CI: 1.983–5.390, P = 0.005), LVEF 35% (OR: 1.945, 95% CI: 1.586–3.492, P = 0.012), IABP usage (OR: 1.365, 95% CI: 1.109–2.196, P = 0.001) and PNI (OR: 0.538, 95% CI: 0.492–0.791, P = 0.033) were determined as independent predictors for mortality. In on-pump cardiac surgery, postoperative mortality is significantly associated with preoperative low PNI, and PNI can be a useful and suitable parameter for preoperative risk evaluation.