A 72-year-old woman with a history of urolithiasis and thoracic compression fracture was referred to our hospital for evaluation of primary hyperparathyroidism. She had osteoporosis, hypercalcemia, hypophosphatemia, and elevated intact PTH. B-mode ultrasound revealed a flat hypoechoic area adjacent to the anterior surface of the thyroid, and power Doppler imaging revealed blood flow in the same area. Three-dimensional ultrasonography revealed an oval hypoechoic area in a coronal section image, measuring 16 mm in the major axis. MIBI scintigraphy showed normal uptake in the thyroid in the acute phase, and nodular remaining uptake in the left lower pole of the thyroid in the delayed phase, whose shape was quite similar to that of the coronal section image. No other ectopic parathyroid gland was found. This hypoechoic area was revealed to be an enlarged parathyroid gland, which was confirmed by postoperative pathology. Although it is well known that the lower parathyroid gland can be found anywhere from the cervical region to the mediastinum due to its embryological procedure, a parathyroid gland on the anterior surface of the thyroid is rarely documented. Three-dimensional ultrasonography was useful for identifying the lesion.