Subacute thyroiditis with coexisting papillary carcinoma is very rare. Here, we report a case of 36-year-old Korean female with both thyroid diseases. She presented with acute onset of anterior neck pain and swelling for 2 weeks. Physical examination, laboratory and image findings suggested subacute thyroiditis. Ultrasonographic examination found a 0.8 cm-sized marked hypoechoic nodule surrounded by multiple irregular hypoechoic lesions and central lymph node metastasis was suspected. Subsequent fine needle aspiration cytology for this nodule showed papillary carcinoma. Two months of oral prednisolone therapy relieved clinical symptoms and normalized her thyroid function. Total thyroidectomy was performed along with central neck dissection and the nodule was confirmed to be papillary carcinoma with one metastatic lymph node among ten dissected nodes, otherwise unremarkable. The other portion of the thyroid showed the typical pathologic features of subacute thyroiditis. Thyroid ultrasonography as an ancillary diagnostic tool in subacute thyroiditis may have clinical implications of finding out hidden malignancy, although the association between two diseases is unclear.