Atypical lipomatous tumor (ALT)/well-differentiated liposarcoma, a relatively common malignant soft-tissue tumor, was first described in 1857 by Rudolf Virchow. It generally occurs in the four limbs and the retroperitoneum and rarely develops in the head and neck region. Here we report a case of an atypical lipomatous tumor in the left submandibular region. A 57-year-old man presented with a slowly enlarging mass in the lower left submandibular region. Initial examinations using computed tomography and magnetic resonance imaging revealed heterogeneous internal patterns; therefore, we performed an open biopsy. Histological examination of the specimen indicated lipoma. In February 2011, the patient underwent tumor excision under general anesthesia. The tumor adhered to the lingual nerve; we divided the tumor into parts to save the nerves and resected as much of the tumor as possible. Based on the histological examination of the resected specimens, an atypical lipomatous tumor was diagnosed. We re-operated in April to excise the remainder of the tumor. We excised it along with the lingual nerve via a mandibular swing approach because the tumor was in contact with the medial pterygoid muscle in the medial aspect of the mandible. No signs of recurrence were noted after seven years.