Extranodal marginal zone lymphoma of mucosa-associated lymphoid tissue (MALT) of the thyroid gland is uncommon. Even though its natural history is not well defined, it is known to be indolent course. We present a case of primary MALT thyroid lymphoma with the serial sonographic findings in the patient presenting as the focal nodule. A 45-year-old woman visited our hospital for neck examination. Initially, fine-needle aspiration cytology in the focal hypoechoic lesion in the left thyroid lobe on ultrasound sonography was performed and consistent with Hashimoto's thyroiditis. However, the results of serial ultrasounds and core-needle biopsy revealed an extranodal marginal zone lymphoma of MALT on 4-year follow-up. Patients with a focal hypoechoic nodule with linear echogenic strands and segmental pattern in the background of Hashimoto's thyroiditis on ultrasonography should undergo careful surveillance for malignancy. Serial sonographic features in this case are meaningful in the understanding of the natural history of the extranodal marginal zone lymphoma of MALT of the thyroid.