The methanol extract from the whole plant of Geum japonicum was found to inhibit the human immunodeficiency virus (HIV-1) protease. Through bioassay-directed fractionation of the extract, a new triterpene acid along with five known triterpene acids, ursolic acid, epipomolic acid, maslinic acid, euscaphic acid, and tormentic acid, were isolated. The structure of the new compound was determined by spectral means including 1H-1H COSY, HMQC, HMBC, and NOE experiments to be 2 alpha, 19 alpha-dihydroxy-3-oxo-12-ursen-28-oic acid (1). Of these compounds, 1, ursolic acid, and maslinic acid showed potent inhibitory activity against HIV-1 protease.