Harpertrioate A (1), an A,B,D-seco-limonoid with a rearranged ring B incorporating exocyclic C-30, was isolated from the EtOAc extract of Harrisonia perforata twigs. Its structure, including absolute configurations, was determined on the basis of spectroscopic data and X-ray crystallography. This compound exhibited biological activities against Alzheimer's disease by reducing Aβ42 and Aβ40 production and shifting APP processing toward nonamyloidogenic pathway. The effect of 1 on the Aβ production was comparable to that of gemfibrozil.