Soft-tissue sarcoma (STS) is a rare tumor that may occur in the upper extremity. Its presentation is delayed by slow growth and lack of symptoms. Lesions are discovered via physical and radiologic examinations, and definitive diagnosis is conducted histopathologically. We present the case of a 63-year-old man with swelling of the dorsum of the arm,nocturnal hyperhidrosis, and weight loss. Radiologic examination revealed possible malignancy and metabolically active left axillary lymph nodes. The tumor was excised, and the defect was covered using a pedicled myocutaneous latissimus dorsi flap. The distal portion of the muscle was attached to the remnant tricep brachii tendon. Postoperative histopathology revealed a highly malignant STS. The patient remained relapse-free after a regimen for adjuvant chemotherapy. Satisfactory aesthetic results and modest elbow extension were evident during the 11-month follow-up. Properly managing upper extremity STS is crucial for preventing recurrence and metastasis.