We present a case of an unusual infectious complication of a ruptured mediastinal abscess after endobronchial ultrasound-guided transbronchial needle aspiration (EBUS-TBNA), which led to malignant pleural effusion in a patient with stage IIIA non-small-cell lung cancer. EBUS-TBNA was performed in a 48-year-old previously healthy male, and a mediastinal abscess developed at 4 days post-procedure. Video-assisted thoracoscopic surgery was performed for debridement and drainage, and the intraoperative findings revealed a large volume pleural effusion that was not detected on the initial radiographic evaluation. Malignant cells were unexpectedly detected in the aspirated pleural fluid, which was possibly due to increased pleural permeability and transport of malignant cells originating in a ruptured subcarinal lymph node from the mediastinum to the pleural space. Hence, the patient was confirmed to have squamous cell lung carcinoma with malignant pleural effusion and his TNM staging was changed from stage IIIA to IV.