Introduction: Ulcerative colitis (UC) is an autoimmune disease commonly managed with immunosuppressants. However, its put UC patients at a higher risk for infections. Therefore, vaccination is an important part of management. Messenger RNA (mRNA) vaccine is a new type of vaccine only widely used against COVID-19. The effect of mRNA vaccines on UC patients on immunomodulatory treatement is relatively unknown. Here, we present the first case of UC flare-up after receiving the SARS-CoV-2 mRNA vaccine. Case description/methods: A 36-year-old female with past medical history of ulcerative colitis, autoimmune pancreatitis presented with intermittent lower abdominal pain and diarrhea for 2 weeks with fever for 1 day. Her last flare-up was 3 years ago before she was put on Tofacitinib. She never required hospitalization. Her symptoms started the second day after she received the second dose of Pfizer COVID-19 vaccine. She had daily 5-10 times of non-bloody diarrhea and abdominal cramps. Her symptoms subsided after 7 days of initial onset. On day 11, she started to have daily up to 15 times bloody bowel movements, along with nausea, vomiting, unable to maintain oral intake. Day 17 after vaccination, patient went to ED after fever of 100.5F. On presentation, she is afebrile, WBC 11.9, CRP 61.8, calprotectin 1840, Clostridioides difficile negative, GI Panel negative. CT showed severe colitis involving the left colon from splenic flexure to rectum. Colonoscopy showed erythematous, granular, hemorrhagic, and ulcerated mucosa in the entire examined colon. Terminal ileum, cecum, ascending, transverse, descending, sigmoid and rectum biopsies were negative for CMV, showed chronic active colitis. Patient received intravenous Methylprednisolone, then transited to oral Prednisone taper and resumed her Tofacitinib. Her symptoms gradually improved. Discussion: To our best knowledge, no report has shown the association of mRNA vaccine and UC flare-up. In general, non-live vaccines are safe for UC patients regardless of immunosuppressants usage. Live vaccines are also contraindicated Tofacitinib users like our patient. In the Pfizer COVID-19 vaccine trial, patients on immunosuppressive therapy were excluded. Given the temporal relationship between the vaccine and her flare-up in the setting of long-term remission history, we are concerned that the stress induced by the mRNA vaccine may increase the risk of flare-up. More studies need to be done to evaluate the safety profile of mRNA vaccine for UC patients on immunosuppressants.Figure 1.: A: Nodular mucosa in second portion of duodenum. B: Multiple ulcers in terminal ileum. C and D: 1.7 cm segment of terminal ileum circumferential bowel wall thickening with mucosal hyperenhancement and submucosal edema.