Background/Aims: Single-institution studies showed that patients presented with more severe diverticulitis and underwent more emergency operations during the coronavirus disease 2019 (COVID-19) pandemic. Therefore, we studied this trend using nationwide data from the American College of Surgeons National Surgical Quality Improvement Program database. Methods: Patients (n = 23,383) who underwent a colectomy for diverticulitis in 2018 (control year) and 2020 (pandemic year) were selected. We compared these groups for differences in disease severity, comorbidities, perioperative factors, and complications. Results: During the pandemic, colonic operations for diverticulitis decreased by 13.14%, but the rates of emergency operations (17.31% vs. 20.04%, P< 0.001) and cases with a known abscess/perforation (50.11% vs. 54.55%, P< 0.001) increased. Likewise, the prevalence of comorbidities, such as congestive heart failure, acute renal failure, systemic inflammatory response syndrome, and septic shock, were higher during the pandemic (P< 0.05). During this same period, significantly more patients were classified under American Society of Anesthesiologists classes 3, 4, and 5, suggesting their preoperative health states were more severe and life-threatening. Correspondingly, the average operation time was longer (P< 0.001) and complications, such as organ space surgical site infection, wound disruption, pneumonia, acute renal failure, septic shock, and myocardial infarction, increased (P< 0.05) during the pandemic. Conclusions During the pandemic, surgical volume decreased, but the clinical presentation of diverticulitis became more severe. Due to resource reallocation and possibly patient fear of seeking medical attention, diverticulitis was likely underdiagnosed, and cases that would have been elective became emergent. This underscores the importance of monitoring patients at risk for diverticulitis and intervening when criteria for surgery are met.