A 44-year-old man whose platelet count had been at the lower limit of the normal range for years visited the urgent care department of our hospital for treatment of a high fever and severe fatigue. The influenza A virus was detected, and the patient therefore received the intravenous antiviral agent, peramivir. One week later, he developed systemic petechial rashes. A peripheral blood examination showed a markedly decreased platelet count (3.0×10(9) cells/L), and the bone marrow findings were compatible with a diagnosis of immune thrombocytopenia (ITP). Furthermore, a drug-induced lymphocyte-stimulating test was positive for peramivir. The thrombocytopenia slowly responded to treatment with oral prednisolone. This case suggests that neuraminidase inhibitors, including peramivir, can elicit or worsen ITP.