A 70-year-old man presented with abdominal swelling. He denied any constitutional symptoms, history of jaundice, infection, or foreign travel. Physical examination was remarkable only for nontender splenomegaly approximately 18 cm below the costal margin. There was no lymphadenopathy, organomegaly, or signs of chronic liver disease. A full blood count, lactate dehydrogenase, blood film, immunoglobulins, renal, and hepatic parameters were normal. Computed tomography (CT) scan confirmed a markedly enlarged septated spleen measuring 26 cm (panel A). Splenectomy revealed a spleen weighing 5166 g and measuring 330 × 230 × 90 mm. Histology showed multiloculated mucinous cysts with features of metastatic gastrointestinal adenocarcinoma (panel B: [upper] hematoxylin and eosin stain, original magnification ×100; [middle] CDX2 stain, original magnification ×10; [bottom] CK20 stain, original magnification ×100). All imaging modalities, including CT, magnetic resonance imaging of the liver, and fluorodeoxyglucose-avid positron emission tomography performed after splenectomy did not identify any primary gastrointestinal malignancy. Endoscopic examination of the entire length of the gastrointestinal tract was unremarkable and tumor markers were not elevated.The patient has remained on a once-every-6-months clinical and CT surveillance program ever since, has not received any form of adjuvant treatment, chemotherapy, or radiation, and has remained cancer-free to date, as he approaches 3.5 years since the initial presentation. Splenomegaly carries broad differential diagnoses and this case shows ectopic adenocarcinoma of unknown primary presenting with massive cystic splenomegaly, a finding not previously reported in the literature.