A 64-year-old woman received a third dose of SARS-CoV-2 mRNA vaccine. On the next day, she developed fever, diarrhea, and abdominal pain and had bloody stools. Total colonoscopy revealed deep ulceration on the whole colon. She was treated with corticosteroid and infliximab and her symptoms improved. She was diagnosed with severe enteritis resembling ulcerative colitis triggered by SARS-CoV-2 mRNA vaccination.