Serous cystic neoplasms of the pancreas are generally considered benign lesions. Malignant counterparts have been occasionally described, and the diagnosis of malignancy is based solely on the presence of synchronous or metachronous metastases to the lymph nodes or liver, direct tumor invasion into adjacent organs, or vascular invasion. However, these malignant serous cystic tumors are lined by benign-appearing glycogen-rich cuboidal cells, which have been morphologically indistinguishable from benign microcystic serous cystadenoma in all the cases reported so far. We report a unique case of microcystic serous cystadenoma giving rise to carcinoma with distinctive histologic features including signet ring-like cells and solid nests. We believe that this case represents the first case of a cytologically malignant neoplasm arising from a benign serous cystadenoma (carcinoma ex microcystic serous cystadenoma).