Background and Objectives: It is crucial to quickly identify those patients who need immediate treatment in order to avoid the various complications related to acute diverticulitis (AD). Although several studies evaluated the neutrophil-to-lymphocyte ratio (NLR) suggesting its predictive value in assessing the severity of AD, results have been inconclusive. Therefore, we aimed to assess the relationship between the neutrophil-to-lymphocyte ratio (NLR), the platelet-to-lymphocyte ratio (PLR), the monocyte-to-lymphocyte ratio (MLR), and systemic immune inflammation (SII) with the severity of AD, the ability to predict the presence or absence of complications, and the recurrence rate, based on the values of inflammatory markers. Materials and Methods: We retrospectively reviewed 147 patients diagnosed with AD between January 2012 to February 2023. Patients were divided into 2 groups, uncomplicated and complicated AD. The characteristics and full blood count between both groups were compared. Results: A total of 65 (44.22%) patients were classified as having complicated AD. The area under the ROC curve (AUROC) defining a Hinchey score ≥ 1b was as follows: SII, 0.812 (95% confidence interval (CI), 0.73 –0.888); NLR, 0.773 (95% CI, 0.676–0.857); PLR, 0.725 (95% CI, 0.63–0.813); MLR: 0.665 (95% CI, 0.542 –0.777). An SII cutoff value of > 1200 marked the highest yield for diagnosing complicated AD, with a sensitivity of 82% and a specificity of 76%. The cumulative recurrence rate was not significantly different in the groups of SII ≥ median vs. SII < median (p = 0.35), NLR ≥ median vs. NLR < median (p = 0.347), PLR ≥ median vs. PLR < median (p = 0.597), and MLR ≥ median vs. MLR < median (p = 0.651). Conclusions: Our study indicates that SII, NLR, and PLR are statistically significant and clinically useful classifying ratios to predict higher Hinchey scores. However, they cannot predict recurrences.