Infectious enteritis is common in patients with inflammatory bowel disease (IBD). This case presented a young woman who underwent remission maintenance therapy for ulcerative colitis (UC). She was suspected of having concomitant Clostridioides difficile and Edwardsiella tarda infections. The patient's symptoms did not improve after initial antibiotic therapy; thus, the treatment strategy was modified to include an intravenous corticosteroid to treat the UC flare-up. Her symptoms significantly improved after corticosteroid administration. This is the first report of a case in which concomitant C. difficile and E. tarda infections occurred with UC flare-up.