Primary thymic mucinous adenocarcinoma is extremely rare; to our knowledge, only 2 cases have been reported to date. We describe a third case of primary mucinous adenocarcinoma of the thymus in a 41-year-old man who presented with an anterior mediastinal mass with subsequent metastasis to the lung. The initial diagnosis was of metastatic mucinous adenocarcinoma, but extensive clinical workup of the patient failed to reveal a primary tumor elsewhere in the body. The specific identification of mucinous adenocarcinoma as a primary thymic neoplasm can be difficult or impossible. Morphologic and immunophenotypic similarities to mucinous adenocarcinomas of the gastrointestinal tract can pose diagnostic challenges for surgical pathologists, especially in small biopsy specimens.