Polymorphous low-grade adenocarcinoma （PLGA） is a rare malignant salivary gland tumor arising most commonly in the palate. We report a case of PLGA in a 72-year-old woman. The patient had a protrusive tumor, measuring 18 × 18 mm, in the left retromolar region. Radiographically, it was located in the submucosa with smooth and shallow resorption of the mandibular bone cortex. A biopsy suggested a benign salivary gland tumor, although a definitive diagnosis was not obtained. The tumor was surgically removed under general anesthesia. It was easily removed from the surface of the mandibular bone. Histopathological examination of the surgical specimen showed a relatively well-demarcated solid epithelial tumor with focal lack of a fibrous capsule. Tumor cells showed a monotonously basaloid appearance characterized by various architectural patterns, including solid, tubular, trabecular, and cribriform ones. Immunohistochemically, the tumor cells were positive for keratin 7, vimentin, and S-100 protein, which indicated myoepithelial characteristics. It was finally diagnosed as PLGA. The postoperative course has been uneventful as of 2 years after surgery.