Trimethoprim–sulfamethoxazole, otherwise known as Bactrim or Septra, is a commonly prescribed antibiotic for soft tissue infections. Drug-induced thrombocytopenia is a rare but serious adverse reaction to sulfonamide antibiotics like Bactrim/Septra. A 34-year-old active duty marine male with no significant past medical history presented with a chief complaint of a rash on his lower extremities. The patient stated that 2 weeks earlier, he was prescribed Bactrim for cellulitis at the site of a new tattoo. The intern noted a petechial rash that was pathognomonic for thrombocytopenia. Laboratory testing confirmed the patient’s thrombocytopenia with platelets of 2,000/μL on initial complete blood count, without pancytopenia or other coagulopathies. The blood smear indicated a profound lack of platelets but otherwise normal cell counts and morphology. In the emergency department, the patient was typed and crossed, platelets were ordered, and hematology–oncology was consulted. Once admitted to the internal medicine ward, he was administered glucocorticoids as well as platelet transfusions. He was monitored for 3 days and discharged with a diagnosis of resolved drug-induced thrombocytopenia. This case illustrates the importance of conducting a thorough review of systems and physical examination in stable and otherwise healthy patients. In this case, the seemingly benign rash was one of the only clinical signs of severe thrombocytopenia, with a high risk of spontaneous bleeding in clinically significant organ systems. It is important to recognize immune thrombocytopenic purpura as a potential complication of Bactrim/Septra, as this antibiotic is widely used by military providers in operational settings.