Leiomyomas are benign mesenchymal tumors derived from smooth muscles. Uterus is the commonest site of origin for leiomyomas; however, unusual growth patterns and locations have been reported posing diagnostic challenges, clinically and radiologically. Histological diagnosis remains the gold standard of diagnosis. Here, we present a 17-year-old female with a 3-month history of chest pain, cough, difficulty in breathing and chest tightness. Chest CT scan showed a 12.3 cm × 14.4 cm × 22.8 cm mass occupying the entire left posterior hemithorax. The patient underwent thoracotomy and recovered well after surgery. Histopathology of the excised specimen confirmed it to be leiomyoma. Despite its rarity, primary mediastinal leiomyoma should be considered in the differential diagnosis of an unexplained mediastinal mass. Its accurate pre-operative diagnosis is difficult. Respiratory compromise and risk of transformation to sarcoma mandate complete surgical resection for a definitive diagnosis and good prognosis.