Undifferentiated pleomorphic sarcoma (UPS) is a high-grade subtype of soft tissue sarcoma (STS) characterized by its aggressive clinical behavior. It typically presents as a rapidly enlarging, asymptomatic subcutaneous mass with minimal or no overlying cutaneous changes. Definitive diagnosis relies on histopathological examination and immunohistochemical analysis. Once confirmed, comprehensive staging—encompassing assessment of the primary lesion, regional lymph nodes, and potential metastatic spread—is essential. Management is generally guided by tumor staging and typically involves en-bloc surgical resection, followed by histopathological review and multidisciplinary consideration of adjuvant therapies, including radiotherapy or chemotherapy. In this report, we present the case of a 78-year-old patient with a previous diagnostic of UPS on the right pectoral area that relapsed accompanied by massive spontaneous pneumothorax and further sepsis.