Small intestinal adenocarcinomas are uncommon neoplasms that are rarely reported in nonhuman primates. These neoplasms are also rare in humans, although they are thought to share a similar pathogenesis with the more common colorectal carcinoma. Herein the authors report the clinical, histologic, immunohistochemical, and molecular characteristics of small intestinal adenocarcinoma in 10 common marmosets (Callithrix jacchus). Retrospective analysis of necropsy records revealed small intestinal carcinoma to be the most common neoplastic cause of morbidity and mortality in aged common marmosets. The average age of affected animals was 6.6 years old, and there was no sex predilection. Nine of 10 (90%) tumors arose within the proximal small intestine near the interface with the duodenum. All cases were characterized by disorganization, loss of polarity, and proliferation of neoplastic epithelial cells along the crypt to midvillous interface. Two of 10 (20%) were defined as carcinoma in situ. Eight of 10 (80%) had some degree of invasion, with lymphatic invasion and lymph node metastasis present in 6 of 10 (60%) animals. Immunohistochemically, 10 of 10 (100%) expressed cytokeratin; 7 of 9 (77%) expressed E-cadherin; and 8 of 9 (88%) expressed beta-catenin. The expression of E-cadherin and beta-catenin was decreased in the cell membrane and increased in the cytoplasm. No Helicobacter-like bacteria were observed via silver stain, and callitrichine herpesvirus 3 was detected by polymerase chain reaction with equal frequency from neoplastic and nonneoplastic intestinal sections. The tumors described in this population illustrate comparable features to human cases of small intestine carcinoma and may serve as a potential animal model for small intestinal carcinomas.