A 53-year-old woman with pulmonary adenocarcinoma developed watery diarrhea 19 weeks after the first administration of nivolumab, an immune checkpoint inhibitor. Computed tomography of the abdomen showed bowel wall thickening serially from the rectum to the ascending colon. Based on the results of stool culture and serum Clostridium difficile toxin assays, bacterial enteritis and Clostridium difficile-associated diarrhea were excluded. Colonoscopy showed diffuse reddening in the entire colon and a superficial ulcer in the descending colon. Colon tissue biopsy revealed cryptitis, crypt abscess, and a decreased number of goblet cells, consistent with ulcerative colitis. However, there was little ductal disarray, which is not typical of ulcerative colitis. Immunohistochemical staining for cytomegalovirus antigen was also positive. We initiated treatment with mesalazine at a dose of 400mg twice daily. One week later, we added valganciclovir at a dose of 900mg twice daily. The stool frequency normalized within 1 week of initiating oral mesalazine. Some patients experience colitis after receiving nivolumab, although it is difficult to determine whether this symptom is caused by nivolumab (i.e., an immune-related adverse effect) or by an inflammatory bowel disease such as ulcerative colitis. This case suggests that there is some overlap between the pathophysiology of nivolumab-induced colitis and ulcerative colitis.