Pleomorphic adenoma is a tumor rarely seen in the breast. To date, only about 70 cases have been reported in the literature. Many reports have used fine-needle aspiration biopsy in the preoperative diagnosis of these lesions. However, pleomorphic adenoma is sometimes mistaken for a malignant tumor because of its cytologic features. In this study, we present a 56-year-old woman with a palpable breast mass. Ultrasound indicated a 15 mm oval-shaped hypoechoic mass with complex echogenicity in the palpable region. Ultrasound-guided core needle biopsy (US-CNB) revealed a pleomorphic adenoma of the breast, which was confirmed through surgical excision. Although pleomorphic adenoma is rarely seen in the breast, results demonstrate the usefulness and accuracy of US-CNB in diagnosing pleomorphic adenoma of the breast.