Leiomyosarcoma, primarily a tumor of smooth muscle origin, frequently originates from the uterus, retroperitoneum, and intra-abdominal region. Rarely, the tumor may arise from the conjunctiva, inferior vena cava, or oral cavity. Here we report a case of a 65-year-old male patient who presented with a swelling in the posterior thigh for six months. The swelling was progressively increasing in size for the last two months. Examination of thigh showed a swelling of 20×30 cm in size, which was firm, non-compressible, immobile, and not transilluminating. CT scan showed no metastasis in the liver, lung, or bone. The histopathology report showed poorly differentiated leiomyosarcoma involving the muscles of the posterior compartment of the left thigh. The tumor was resected, and the patient was referred to rehabilitation clinic. Early diagnosis of such cases is essential to improve the outcome in patients as these tumors can metastasize early.