In spite of the critical role of the natural products in drug discovery, surprising little attention has been placed on endangered and rare plant species that could play a pivotal role in pharmaceutical and fiber development. Protein tyrosine phosphatase-1B (PTP1B), which blocks insulin signaling, has been gaining interest to be a promising therapeutic target for the treatment of type 2 diabetes mellitus. Bioassay-guided fractionation on the leaves of Rhododendron brachycarpum G. Don (Ericaceae) yielded seven PTP1B inhibitory triterpenoids, including a new triterpene, rhododendric acid A (1). Their PTP1B inhibitory potency and their lipophilicity were investigated to provide a feasible scaffold that may overcome the innate limitation of the previously reported PTP1B inhibitors.