Whole body scintigraphy with I-131 has a high sensitivity for detecting thyroid tissue. I-131 radioactive iodine accumulation has been reported in mature cystic teratoma (MCT) accompanied by struma ovarii, which contains thyroid tissue, and a teratoma has been reported to contain various tissue types. Here we present a rare case of positive radioactive iodine uptake in MCT with mucinous cystadenoma in a 50-year-old patient. I-131 radioactive iodine uptake is variable, and the correct diagnosis and appropriate management of patients should be ensured.