Phytochemical investigations of leaves and twigs from Garcinia oligantha Merr. resulted in the isolation of five undescribed triterpene derivatives (1–5) and six known analogs (6–11). Their structures were determined based on extensive spectroscopic data and high-resolution mass spectra analyses. Compounds 1–11 were tested for their in vitro cytotoxicity against three human cancer cell lines (HeLa, HepG-2, and MCF-7). Compounds 1, 2, 8, and 11 exhibited broad and significant cytotoxicity against the tested cell lines with IC50 values ranging from 5.04 to 21.55 μM. Compounds 5 and 9 showed cytotoxicity against HeLa and MCF-7 with IC50 values ranging from 13.22 to 19.62 μM. The preliminary structure–activity relationship for the 11 isolated compounds is also discussed.