The major saponin named thornasteroside A (5) has been isolated from the whole body of starfish Acanthaster planci L. (crown of thorns). In order to facilitate the structural study, thornasteroside A (5) has been converted to a pentaglycoside (designated as dsp-1), which corresponds to a desulfated pregnane-type derivative of 5. On the bases of chemical reactions and physicochemical evidence, the structure of dsp-1 has been elucidated as 10. Based on the accumulated evidence in this and the preceding papers, the structure of thornasteroside A has been established as 20-hydroxy-6α-O-{β-D-fucopyranosyl (1→2)-β-D-galactopyranosyl (1→4)-[β-D-quinovopyranosyl (1→2)]-β-D-xylopyranosyl (1→3)-β-D-quinovopyranosyl}-5α-cholest-9 (11)-en-23-on-3β-yl sodium sulfate (5).