Alismanin A (1), a novel aromatic triterpenoid with a C34 skeleton, was isolated from Alisma orientale together with a rearranged nor-triterpenoid (2) and a 13,17-seco triterpenoid (3). Their structures were determined by a combination of HRESIMS, 2D NMR spectra, electronic circular dichroism (ECD), theoretical calculations, and X-ray diffraction analysis. Compounds 1 and 2 displayed significant activation effects on pregnane X receptor (PXR) at 10 nM. A plausible biosynthetic pathway for 1-3 is also discussed.