Thymoma is the most common primary anterior mediastinum mass with various clinical manifestations, and one of the manifestations is pericardial effusion. While pericardial effusion in thymoma is usually serous, it can become purulent when an infection occurs in a nearby organ, albeit rare. In this report, we present a rare case of a 27-year-old woman who had purulent pericarditis secondary to an advanced thymoma. The patient came to the emergency department with the chief complaints of worsening chest discomfort, non-productive cough, and fever in the past 2 weeks. The patient was diagnosed with thymoma 5 months prior. Based on the examinations, it was discovered that the patient had pericarditis. After the pericardiocentesis was performed and the fluid was examined, the patient was diagnosed with purulent pericarditis secondary to thymoma. The patient was then treated with intravenous antibiotic and pericardial drain. Unfortunately, the patient's condition deteriorated, and the patient died on the fifth day of hospitalization. This case highlights an infrequent but potentially life-threatening complication of thymoma. In addition, thymic pathologies should be included as a rare etiology in the differential diagnosis of purulent pericardial effusion.