Cases of simultaneously occurring medullary thyroid carcinoma (MTC) and lymphoma are extremely rare. An 84-year-old woman visited the hospital due to dyspnea, resulting from rapidly aggravated enlarged neck mass. Ultrasonography revealed two lesions in the thyroid and they were diagnosed as concurrent medullary thyroid carcinoma and diffuse large B cell lymphoma after total thyroidectomy. A few cases simultaneously diagnosed with MTC and systemic lymphoma have been reported. However, the coexistence of MTC and primary thyroid lymphoma is extremely rare.