RATIONALE: Ulcerative colitis (UC) is a chronic inflammatory bowel disease that primarily affects the colon. Although UC is classically limited to the colonic mucosa, rare cases of small bowel involvement have been reported. UC-associated small bowel inflammation is an uncommon complication, most frequently described following colectomy with ileal pouch formation. However, emerging evidence suggests that it can also occur in patients without prior surgery. PATIENT CONCERNS: A 27-year-old female with a 6-year history of UC, well-controlled with mesalazine, developed intermittent periumbilical and left upper quadrant abdominal pain 5 years after disease onset. DIAGNOSES: Small bowel endoscopy revealed the presence of multiple ulcers in the horizontal duodenum and proximal jejunum. Histopathological examination showed loss of villous epithelium, widespread hemorrhage in the lamina propria, flattened crypts, reduced crypt density, and neutrophilic infiltration with associated bleeding. INTERVENTIONS: After the diagnosis of UC-associated small bowel inflammation was confirmed through imaging, endoscopy, and histopathological examination, the patient did not respond to steroid therapy. Treatment with infliximab resulted in significant clinical improvement. OUTCOMES: The patient's abdominal pain and inflammatory markers significantly improved after infliximab therapy. Follow-up endoscopy confirmed mucosal healing. She remained in steroid-free clinical remission during follow-up. LESSONS: UC-associated small bowel inflammation is typically observed following colectomy; however, this case underscores that it can also occur in patients without prior colectomy. The condition often presents with unexplained abdominal pain, nausea, and vomiting, symptoms that may not be attributable solely to UC. For patients with extraintestinal symptoms who do not respond to steroid treatment, biologic therapies, such as infliximab, may provide an effective therapeutic option.