A 2.5-yr-old, intact male Japanese macaque (Macaca fuscata) was observed to have a thickened ileum during exploratory laparotomy. Lawsonia intracellularis–associated proliferative enteritis was diagnosed using histopathology (Warthin–Starry stain), immunohistochemistry, and polymerase chain reaction analysis of the ileal biopsy. The animal developed transient diarrhea and severe hypoproteinemia 16 days after surgery but recovered with intensive treatment using azithromycin. Given the fact that very specific tests are required for identifying this organism, L. intracellularis may be underdiagnosed in nonhuman primates.