Pulmonary artery angiosarcoma (PAA) is an extremely rare malignancy that arises from endothelial cells. The pulmonary trunk is the most common anatomic site of these tumors. However, so far, there have only been a few examples of PAA reported. PAAs are aggressive and have a bad prognosis, thus early diagnosis is critical. In this study, we present the case of a 27-year-old male who was initially diagnosed with acute pulmonary thromboembolism (PTE) before being diagnosed with chronic thromboembolic pulmonary hypertension (CTEPH). Pulmonary endarterectomy was performed, and PAA was diagnosed histopathologically. The patient died three months after diagnosis.