Background: Due to the known poor prognosis of complicated intra-abdominal infections (cIAIs), a clinically usable predictive model of cIAI outcomes may be used to identify the high-risk patients and encourage appropriate management. Objectives: The aim of the current work was to evaluate and compare Mannheim Peritonitis Index (MPI) and Sepsis Severity Score (SSS) validity in predicting the cIAIs related mortality based on score parameters. Patients and Methods: This observational prospective study included a total of 143 patients with acute abdomen caused by intra-abdominal sepsis, attending at Department of General Surgery, Zagazig University Hospitals. during the period from December 2018 to December 2019. The enrolled patients were evaluated by two different scores (SSS and MPI) without any interference in management decisions or a plan and comparing each score. Results: For SSS and MPI, respectively, the optimal cut-off points discovered from the curve were 9.5 and 24.5. Both scores according to the ROC curve demonstrated an excellent mortality prediction, with the area under the curve being outstanding for both scores (AUC > 0.7). The MPI total accuracy (67.13%) was lower than that of SSS (75%), but MPI produced true positives indicating higher sensitivity than SSS, which produced more specificity (true negative). Between MPI and SSS, there was a fair degree of agreement (kappa agreement = 0.603) and a statistically significant moderately positive correlation (r=o.562). Conclusion: It could be concluded that MPI score is more sensitive than WSES-SSS in the prediction of mortality, however, WSES-SSS is more specific for the prediction of intra-abdominal sepsis related mortality.