We herein report a case of concurrent immune thrombocytopenia (ITP) and ulcerative colitis (UC) that achieved remission following mesalazine treatment. A 16-year-old girl presented with severe thrombocytopenia, abdominal pain, and bloody stool. She was initially diagnosed with ITP and then was treated with prednisolone, resulting in an immediate improvement of symptoms. Upon tapering the steroids, the symptoms recurred, thus leading to a subsequent diagnosis of UC via colonoscopy. Treatment with mesalazine promptly induced the remission of both ITP and UC, which was sustained. We reviewed 24 previously documented cases in which the simultaneous flares of UC and ITP were successfully managed.