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-β-d-glucopyranosyl presenegenin 28-O-β-d-xylopyranosyl-(1 → 3)-β-d-xylopyranosyl-(1 → 4)-[β-d-apiofuranosyl-(1 → 3)]-α-l-rhamnopyranosyl-(1 → 2)-[4-O-p-methoxycinnamoyl]-[α-l-rhamnopyranosyl-(1 → 3)]-β-d-fucopyranosyl ester (1). Three known triterpenoid saponins (2-4) were identified on the basis of spectroscopic data.