Chemical investigation on the 75% ethanol extract of the roots of Streptocaulon juventas afforded two new cardiac glycosides, digitoxigenin 3-O-[O-β-D-glucopyranosyl-(1 → 4)-2-O-acetyl-β-D-digitalopyranoside] (1) and periplogenin 3-O-[O-β-D-glucopyranosyl-(1 → 4)-O-β-D-glucopyranosyl-(1 → 4)-β-D-cymaropyranoside] (2), and thirteen known cardenolides. Structures were elucidated by spectral methods. This is the first report of the isolation of compounds 3, 10, 14, and 15 from plants of the Streptocaulon genus, while 4, 11, and 12 are hitherto unreported from Streptocaulon juventas. All the compounds were in vitro evaluated for their cytotoxic activities against the A549 cell line, and seven effective cardiac glycosides were screened against the PC-9 cell line by WST assay, which also showed strong antiproliferation activities. Moreover, the characteristic morphological changes in PC-9 cells treated with cardenolides indicated cell inhibition due to apoptosis. These results revealed that these compounds possessed potential antitumor activities.