Objective: To investigate the relationships between red cell distribution width (RDW), other inflammation-related markers and clinical features with the clinical and radiological severity of diverticulitis. Methods: This retrospective cohort study included 250 patients diagnosed with acute diverticulitis. Radiological diverticulitis severity was determined by the Hinchey classification. Clinical diverticulitis severity was determined based on the treatment applied (invasive versus conservative/medical treatment). Results: High platelet count (p = 0.001) and high CRP (p < 0.001) were independently associated with the Hinchey Class II-IV. Need for invasive treatment was independently associated with presence of Hinchey stage II-IV (p < 0.001) and high (> 13.75) RDW (p < 0.001). With a cut-off value of > 13.75, RDW was able to predict patients requiring invasive treatment [Sensitivity = 75.86%, Specificity = 63.87%, AUC (95% CI) = 0.657 (0.580–0.734), p < 0.001]. Conclusiosn: RDW, which is an inexpensive and readily-available parameter, may be a supportive measure in the prediction of the clinical severity of diverticulitis. Together with other clinical and laboratory data, RDW could help identify patients with worse prognosis, facilitating appropriate decisions and precautions in their management.