We present the case of a 58-year-old patient presented with a spontaneous right supracondylar fracture. The initial bone biopsy, highlighted the defining histopathological (HP) elements for a leiomyosarcoma (LMS), initially considered a metastasis. The complex imaging examinations did not reveal another tumor, so the final diagnosis was primary bone LMS. Final treatment was a wide tumor resection and reconstruction with a knee tumor prosthesis, preceded and followed by three cytostatic cycles (Doxorubicin 75 mg∕m²). The HP examination has confirmed the previous diagnosis. The key microscopic features for the diagnosis of bone LMS was: malignant mesenchymal proliferation composed of intersecting fascicles of cells with eosinophilic, fibrillary cytoplasm and pleomorphic, elongated, blunt-ended, cigar-shaped nuclei of variable sizes; variable mitotic count; presence of tumor necrosis and stroma with changes that include hyalinization, myxoid change, with absence of chondroid or osteoid matrix; diffuse positivity for smooth muscle immunohistochemical markers: smooth muscle actin, desmin, h-caldesmon. At 12 months after the tumor resection, the patient is in good condition without any sign of local recurrence or metastatic disease. LMS represents a type of soft tissue sarcoma (STS), a variant of the spindle cell sarcomas, accounting for about 7% to 10% of all STS. Bone LMS can be primary or secondary; the primary variant is very rare, representing a very small percentage (around 0.7%) of all primary malignant bone tumors, according to the literature data. Very few cases are presented in the literature; the management of this kind of tumor is controversial, especially regarding the chemo- and radiotherapy.