We describe herein a case of thyroid lymphoma of the mucosa-associated lymphoid tissue (MALT) with monoclonal gammopathy (IgG kappa), occurring in a woman who was exposed to atomic-bomb (A-bomb) radiation 1.5 km from the explosion of the hypocenter when she was 31 years old. Her estimated radiation dose was 0.2 Gy. A total thyroidectomy resulted in the disappearance of monoclonal gammopathy, thus indicating extramedullary monoclonal gammopathy of the thyroid. Three patterns of protein electrophoresis, i.e., normal, the appearance of monoclonal immunoglobulins, and their disappearance after the total thyroidectomy, were consecutively recorded. The utility of checking of the monoclonal immunoglobulin levels after a thyroidectomy due to lymphoma of the MALT to monitor recurrence is therefore suggested. This lymphoma is most likely not related to the patient's A-bomb radiation exposure.