This report describes the case of a 53-year-old female, who presented with a painless swelling in the left submandibular region which had gradually increased in size. Thirty years ago, this patient underwent subtotal thyroidectomy at another hospital and her thyroid function subsequently appeared normal. She presented no symptoms of hypothyroidism when she visited our hospital. A clinical diagnosis of a benign tumor arising from the submandibular gland was made with enhanced CT scan, MRI and technesium scintigraphy. The submandibular mass was surgically excised and the resected tissue revealed a well-delineated, encapsulated, solid and reddish-brown colored mass. Histological examination confirmed ectopic thyroid tissue with partial adenomatous goiter including vacuolated rich colloid in the thyroid follicles which suggested compensatory hyperplasia. Because this patient presented with post-operative hypothyroidism, we concluded that this ectopic lesion had continued to secrete thyroid hormone. Ectopic thyroid tissue should be considered in the differential diagnosis of swellings involving the submandibular area, especially if the ectopic tissue would be the only functioning thyroid tissue.