Primary thymic marginal zone B-cell lymphoma (MZBL) of mucosa-associated lymphoid tissue (MALT)-type is a very rare disease with distinct clinicopathologic features. I herein report a rare case of primary thymic MZBL of MALT-type arising in the thymus in a patient with Sjögren's syndrome and rheumatoid arthritis. A mediastinal mass was detected by computerized tomography in a 43-yr-old Korean woman with a history of Sjögren's syndrome and rheumatoid arthritis and the thymus was resected through median sternotomy. The solid and nodular tumor (7x6x3cm) was confined in the thymus. Histologically, the lymphoid infiltrate comprised monotonous centrocyte-like cells with monocytoid cells, small lymphocytes, and plasma cells. Prominent lymphoepithelial lesions were formed by centrocyte-like cells infiltrating the Hassall's corpuscles. Immunohistochemically, the tumor cells were positive for CD20, CD79a, and bcl-2 and negative for CD3, CD5, CD10, CD23, and bcl-6. IgA and kappa light chain restriction were also found in plasma cells in the tumor. Sjögren's syndrome and rheumatoid arthritis are known to be associated with MALT lymphoma and were considered to play an important role in the development of malignant lymphoma in this patient.