<p>A 65-year-old man with polycystic kidney disease who had undergone living-donor kidney transplantation presented with fever and left flank pain and was diagnosed with renal cyst infection. Combination antimicrobial therapy with levofloxacin, clindamycin, meropenem, and teicoplanin was initiated. His platelet count was 257,000/µL on day 5; however, it abruptly declined overnight to below the lower limit of detection (&lt; 2,000/µL). He subsequently developed a massive retroperitoneal hematoma, presumably due to hemorrhage from the infected renal cyst, and required emergency selective arterial embolization. Drug-induced immune thrombocytopenia was strongly suspected, and all recently initiated medications were discontinued. He was treated with high-dose intravenous dexamethasone and intravenous immunoglobulin, leading to a transient recovery of the platelet count. However, thrombocytopenia recurred during prednisolone tapering. Although drug-dependent platelet-reactive antibodies were not detected in the patient’s serum, teicoplanin was considered a plausible causative agent based on pharmacokinetic considerations and previous reports; however, the causative drug could not be definitively identified. This case illustrates fulminant thrombocytopenia highly suggestive of drug-induced immune thrombocytopenia in which the platelet count fell to an undetectable level overnight. In patients receiving multiple drugs, clinicians should remain vigilant for drug-induced immune thrombocytopenia and should closely monitor platelet counts during treatment.</p>