Four compounds (1,5,7, and8) were first isolated from the genusBelamcanda Adans. nom. conserv., and six known compounds (2–4,6,9, and10) were isolated from therhizomeofBelamcanda chinensis(L.) DC. Their structures were confirmed by spectroscopic data. Herein, compounds1–10were rhapontigenin, trans-resveratrol, 5,7,4′-trihydroxy-6,3′,5′-trimethoxy-isoflavone, irisflorentin, 6-hydroxybiochannin A, iridin S, pinoresinol, 31-norsysloartanol, isoiridogermanal, and iristectorene B, respectively. All compounds were evaluated for their antiproliferative effects against five tumor cell lines (BT549, 4T1, MCF7, MDA-MB-231, and MDA-MB-468). Among them, compound9(an iridal-type triterpenoid) showed the highest activity against 4T1 and MDA-MB-468 cells. Further studies displayed that compound9inhibited cell metastasis, induced cells cycle arrest in the G1 phase, exhibited significant mitochondrial damage in 4T1 and MDA-MB-468 cells including excess reactive oxygen species, decreased mitochondrial membrane potential, and induced 4T1 and MDA-MB-468 cell apoptosis for the first time. In summary, these findings demonstrate that compound9exerts promising potential for triple-negative breast cancer treatment and deserves further evaluation.