Background: The Neutrophil-to-Lymphocyte Ratio (NLR) is an emerging biomarker for the diagnosis and severity assessment of acute appendicitis. This study evaluates the predictive value of NLR in patients undergoing appendicectomy. Patients and Methods: This cross-sectional retrospective study included 856 patients who underwent open appendicectomy for suspected acute appendicitis between 2015 and 2023. Preoperative clinical assessments, standard laboratory tests, and imaging techniques such as ultrasonography and CT were used for diagnosis. An automated hematologyanalyzer measured neutrophil and lymphocyte percentages to calculate NLR. Patients were divided into two groups: Group A (acute appendicitis) with subgroups A1 (non-complicated) and A2 (complicated), and Group B (normal appendix). Results: Of the 856 patients, 821 (95.9%) had acute appendicitis (Group A) and 35 (4.1%) had a normal appendix (Group B). Group A had a significantly higher mean NLR (7.29 ± 6.30) compared to Group B (5.30 ± 4.70) (p < 0.001). The NLR cutoff value of 4.8 yielded a sensitivity of 66.63% and specificity of 51.43% for distinguishing acute appendicitis. Within Group A, non-complicated appendicitis (A1) had a mean NLR of 7.79 ± 8.68, while complicated appendicitis (A2) had a mean NLR of 6.99 ± 5.92. The cutoff value of 5.8 for subgroup differentiation resulted in a sensitivity of 73.13% and specificity of 49.05%. Conclusions: The Neutrophil-to-Lymphocyte Ratio is a valuable predictor of acute appendicitis and its severity. An NLR cutoff of 4.8 effectively distinguishes between acute appendicitis and a normal appendix, while a cutoff of 5.8 differentiates between complicated and non-complicated appendicitis.