In this study, we report the isolation from the stem of Syzygium kusukusense of five triterpenes, 2α-hydroxybetulinic acid (1), betulinic acid (2), platanic acid (3), ursolic acid (4), and hyptatic acid A (5). All were identified for the first time from this indigenous plant of Taiwan. Assessment of the cytotoxic activities of these compounds against a panel of human tumor cell lines, including MCF-7 breast, PC-3 prostate, and SCC2095 oral squamous cell cancers, revealed the high potency of compounds 1 (IC50, 5.7 - 7.6 μM) and, especially, 4 (IC50, 1.7 - 3.7 μM) in suppressing cell viability, which warrants further mechanistic investigations.