Bioassay guided fractionation and separation of the EtOH extract of the kernels of Palaquium formosanum against PC-3 cells via Sephadex LH-20 and reverse phase C-18 columns led to the isolation of 13 protobassic saponins. One of these saponins is new and was characterized as 3‴-O-rhamnopyranosyl-arganin C, a bisdesmoside of 16α-hydroxyprotobassic acid at the C-3 and C-28 positions. The structures of these compounds were determined on the basis of 1D NMR (1H, 13C), 2D NMR (1H–1H COSY, HSQC, HMBC, and NOESY), and selectively excited 1D TOCSY spectroscopic analyses and MS data, and comparison with literature data. Bioassay of these compounds and five additional compounds, isolated from Planchonella obovata leaf, against PC-3 prostate cancer cells indicated arganin C to be the most potent one with the IC50 value of 13.8 μM. Some structure and activity relationships were also drawn.