A new triterpenoid saponin, tenuifoside A, was isolated together with three known triterpenoid saponins 2, 3, and 4 from the roots of Polygala tenuifolia Willd. With the help of chemical and spectral analyses (IR, MS, 1D-NMR, and 2D-NMR), the structure of the new saponin was elucidated as 3-O-beta-d-glucopyranosyl presenegenin 28-O-beta-d-xylopyranosyl-(1 --> 3)-beta-d-xylopyranosyl-(1 --> 4)-[beta-D-apiofuranosyl-(1 --> 3)]-alpha-L-rhamnopyranosyl-(1 --> 2)-[4-O-p-methoxycinnamoyl]-[alpha-l-rhamnopyranosyl-(1 --> 3)]-beta-d-fucopyranosyl ester (1). Three known triterpenoid saponins (2-4) were identified on the basis of spectroscopic data.