Objectives: Cytomegalovirus (CMV) colitis is generally diagnosed in immunocompromised patients. It is rare for patients who are not immunocompromised to develop CMV colitis. Cases of CMV colitis in patients with inflammatory bowel disease have also been reported. We encountered a case of CMV colitis with a new diagnosis of severe ulcerative colitis and demonstrated the importance of suspecting ulcerative colitis in immunocompetent patients with CMV colitis.Patient: A 78-year-old woman was hospitalized with fever and diarrhea that had lasted for a month. Colonoscopy revealed continuous diffuse edema, mucosal redness, and multiple punched-out ulcers with bleeding, suggesting cytomegalovirus (CMV) colitis, although she was not immunocompromised. Immunohistochemical staining revealed CMV-positive cells, and CMV colitis was diagnosed. One month later, a colonoscopy was conducted owing to persistent symptoms despite initiating the prescribed antiviral drug. A complete loss of vascular pattern, easy bleeding of the crude mucosa, and exacerbation of multiple punched-out ulcers were observed. She was diagnosed with severe ulcerative colitis. The symptoms of ulcerative colitis disappeared with prednisolone and 5-amino salicylic acid treatment.Conclusion: Ulcerative colitis should be suspected in immunocompetent patients with CMV colitis.