T-cell/histiocyte-rich large B-cell lymphoma (THRLBCL) is a rare subtype of diffuse large B-cell lymphoma, and has been reported to mainly affect lymph nodes with advanced Ann Arbor stage. We present a 69-year-old woman with a past history of chronic thyroiditis, who had suffered from rapidly-growing thyroid tumor. No other lymph nodes swelling or extranodal lesions were detected. She promptly underwent surgery with resection of the thyroid tumor, and the diagnosis of THRLBCL was established pathologically. She was successfully treated by standard rituximab-containing chemotherapy. To our knowledge, this is the first case report of THRLBCL exclusively arising in the thyroid.