Euonymus alatus (Celastraceae) has been used as an anticancer agent in Korean traditional medicine. However, the potential bioactive contributors to the anticancer effects have not been thoroughly studied. Our screening test revealed that the MeOH extract of E. alatus twigs exhibited significant cytotoxicity against A549, SK-OV-3, and SK-MEL-2 cell lines. A bioassay-guided separation of the MeOH extract of E. alatus twigs resulted in the isolation and identification of 14 triterpenes as main phytochemicals. The structures of the compounds were elucidated on the basis of spectroscopic evidence as lupeol (1), betulin (2), 3β,28,30-lup-20(29)-ene triol (3), lupenone (4), betulone (5), 28,30-dihydroxy-3-oxolup-20(29)-ene (6), messagenin (7), glut-5-en-3β-ol (8), maslinic acid (9), hederagenin (10), 3-oxo-11α-methoxyolean-12-ene (11), 3β-hydroxy-1-oxo-olean-12-en-28-oic acid (12), ursolic acid (13), and 2a-hydroxy- ursolic acid (14). Of these compounds, 3, 6-8, and 10-14 were isolated for the first time from this plant. All isolated triterpenoids had consistent antiproliferative activities against A549, SK-OV-3, SK-MEL-2, and HCT-15 cell lines. Compounds 2, 5, and 7 showed significant cytotoxicity against all four cell lines tested, with IC50 values of 3.26-8.61 µM.