A 44-year-old male with a mediastinal mass measuring 3.5 × 3.5 × 3 cm was diagnosed with papillary adenocarcinoma of the thymus. Other origins of papillary adenocarcinoma were excluded by clinical, imaging, and immunocytochemical methods before assuming this diagnosis. Residual thymus was seen under the microscope. Focal CD5 immunoreactivity was present. There was no associated thymoma. The patient underwent surgery, radiotherapy, and chemotherapy. He disclosed systemic recurrence at 18 months (subcutaneous nodule). He is alive after 24 months of follow-up with active disease. There had been only 7 cases of this rare entity published before.