TAFRO syndrome, a rare systemic inflammatory disease, can lead to multiorgan failure without appropriate treatment. Although thrombocytopenia is frequently seen in patients with TAFRO syndrome, little is known about its pathogenesis. Moreover, while recent studies have reported the presence of an anterior mediastinal mass in some patients, the pathological status of this remains unclear. Here, we report a case of fatal bleeding in a patient with TAFRO syndrome accompanied by an anterior mediastinal mass. A 55-year-old female was transferred to our hospital with a 2-week history of fever, epistaxis, and dyspnea. Laboratory tests revealed severe thrombocytopenia, computed tomography (CT) showed pleural effusions, and bone marrow biopsy revealed reticulin myelofibrosis. We suspected TAFRO syndrome, but the CT scan showed an anterior mediastinal mass that required a biopsy to exclude malignancy. She soon developed severe hemorrhagic diathesis and died of intracranial hemorrhage despite intensive treatment. She had multiple autoantibodies against platelets, which caused platelet destruction. An autopsy of the mediastinal mass revealed fibrous thymus tissues with infiltration by plasma cells. Our case suggests that thrombocytopenia could be attributed to antibody-mediated destruction and could be lethal. Hence, immediate treatment is imperative in cases of severe thrombocytopenia, even when accompanied by an anterior mediastinal mass.