A 79-year-old man with smoldering multiple myeloma (MM) developed sudden severe thrombocytopenia and active bleeding. Since anti-GPIIb/IIIa autoantibodies were detected on the platelet surface, thrombocytopenia due to an autoimmune etiology was diagnosed. Although neither steroids nor intravenous immunoglobulins provided improvement, bortezomib provided rapid normalization of the platelet counts with disappearance of the anti-GPIIb/IIIb autoantibodies. MM is rarely associated with immune thrombocytopenic purpura (ITP). Although the underlying pathogenesis of MM and ITP remains elusive, anti-myeloma drugs might be beneficial.