Purpose: Patients undergoing major emergency abdominal surgery have a high mortality rate. Preoperative risk prediction tools of in-hospital mortality could assist clinical identification of patients at increased risk and thereby aid clinical decision-making and postoperative pathways. The aim of this study was to validate the preoperative score to predict mortality (POSPOM) in a population of patients undergoing major emergency abdominal surgery. Methods: POSPOM was investigated in a retrospectively collected cohort of patients undergoing major emergency abdominal surgery at a Danish University Hospital from 2010 to 2016. Predicted in-hospital mortality by POSPOM was compared to observed in-hospital mortality. Calibration was assessed by Hosmer–Lemeshow goodness-of-fit and calibration plot. Discrimination was assessed by area under the receiver operating characteristic curve and accuracy was assessed with Brier score. Results: The study included 979 patients (513 females) with a median age of 64 (IQR 55–77) years. The majority of patients underwent open surgery (94.5%). The observed in-hospital mortality rate was 10.9%. The estimated mean in-hospital mortality rate by POSPOM was 6.7%. POSPOM showed a good discrimination [AUC 0.82 (95% CI 0.78–0.85)] and an excellent accuracy [Brier score 0.09 (95% CI 0.07–0.10)]. However, a poor calibration was found (p < 0.01) as POSPOM underestimated in-hospital mortality. Conclusions: POSPOM is not an ideal prediction model for in-hospital mortality in patients undergoing major emergency abdominal surgery due a poor calibration.