We report herein the rare case of a patient with dendriform pulmonary ossification (DPO) who developed spontaneous pneumothorax. A 33-year-old male with a history of bronchial asthma presented with pneumothorax of the left lung. An intraoperative inspection revealed no findings of bullae in the entire left lung, but inflammatory pleural changes were identified on the interlobular surface of the left lower lobe. In addition, hard, twig-like configurations were clearly palpable in the subpleural parenchyma and were resected. A histological examination showed acicular bone formations containing myeloid tissue and marrow fat in the lung. DPO was thus diagnosed, and the bony spines were considered to have caused a rupture of the elastic fiber layer of the visceral pleura. DPO may thus have been directly responsible for the pneumothorax in this case.