Pulmonary embolism in the pediatric population is rare but could be a cause of significant morbidity and mortality. Pleural effusion often complicates pulmonary embolism but it is usually small to moderate in size. We report the case of a 16-year-old male patient with massive pleural effusion as a delayed complication of pulmonary embolism. He presented with pleuritic chest pain, dyspnea, and deep venous thrombosis in the leg, the risk factors for which were obesity and a hypercoagulable state due to a factor V Leiden heterozygous mutation. The patient received anticoagulation treatment with enoxaparin. Three weeks after discharge home, he was readmitted for new-onset chest pain and dyspnea. He was found to have massive left pleural effusion without recurrent pulmonary embolism. The effusion was drained because it compromised the patient's respiratory function. The mechanism for pleural effusion in pulmonary embolism is unclear but may involve increased pulmonary capillary permeability due to inflammatory mediators released from platelet-rich thrombi and pulmonary ischemia distal to the vascular obstruction. This case highlights the potential for late pleural effusion progression in pediatric pulmonary embolism and the need to manage large effusions to prevent respiratory compromise.