Leiomyosarcoma is a rare malignant tumor originating from smooth muscle cells, most commonly affecting the uterus, retroperitoneum, and extremities, while primary involvement of the abdominal wall is exceptionally uncommon. We report the case of a 55-year-old woman who presented with a progressively enlarging mass on the left abdominal wall over six months. Imaging studies revealed a 10 × 3.9 × 13 cm well-defined lesion arising from the external oblique muscle, without intra-abdominal extension or distant metastasis. A core needle biopsy demonstrated spindle-shaped tumor cells with elongated, hyperchromatic nuclei and eosinophilic cytoplasm. Immunohistochemical analysis showed positivity for smooth muscle actin (SMA), h-caldesmon, vimentin, CD68, and CD45, and negativity for pancytokeratin (AE1/AE3), desmin, CD34, S100, myogenin, MDM2, and EMA, confirming the diagnosis of a pleomorphic leiomyosarcoma rich in osteoclast-like giant cells (FNCLCC grade III). The patient underwent wide local excision with tumor-free (R0) margins. She did not receive adjuvant radiotherapy due to financial constraints. Histopathological examination of the surgical specimen confirmed the diagnosis and clear resection margins. At six months of follow-up, there was no evidence of local recurrence or metastasis. This case highlights the rarity of primary leiomyosarcoma of the abdominal wall and underscores the importance of imaging, histopathology, and immunohistochemistry for diagnosis. Complete surgical excision with negative margins remains the mainstay of treatment, and long-term follow-up is essential due to the risk of recurrence and metastasis.