Tumors are rare causes of calf pain and usually present insidiously. A 69-year-old woman developed sudden severe pain and a hard palpable mass in her left calf that persisted for 6 months without a history of trauma. Although a myofascial trigger point was initially suspected, subsequent ultrasonography revealed two well-defined heterogeneous masses in the calf muscle. Magnetic resonance imaging revealed a multi-lobulated mass involving the soleus, tibialis posterior muscle, and deep peroneal neurovascular bundle, suggesting a soft tissue sarcoma. Fluorine-18-fluorodeoxyglucose positron emission tomography revealed a heterogeneous hypermetabolic lesion in the left calf, suggesting malignancy. The patient received an incisional biopsy of her left calf lesion and was diagnosed with leiomyosarcoma. The patient underwent a wide excision with partial fibulectomy of the left calf and received chemotherapy for metastasis in the left upper lobe of the lung. Although rare, soft tissue sarcoma should be considered in the differential diagnosis if calf pain and a palpable mass persist despite conservative management, and imaging studies are essential for distinguishing tumors from other causes of calf pain.