This case report describes a 38-year-old female patient with a 3-year history of multiple sclerosis who developed rituximab-induced pancolitis, possibly representing a new onset of inflammatory bowel disease. The patient presented with bloody diarrhea, epigastric pain, fever, and general malaise. Laboratory testing revealed elevated acute inflammation markers, and endoscopy showed deep ulcerations and severe perianal disease. The patient was treated effectively with corticosteroids. Monthly doses of ustekinumab have been administered during follow-up due to perianal disease that has remitted. Rituximab was discontinued and ozanimod was initiated with clinical and analytical stability to date.