A new phenolic derivative, acidumonate (1), together with a synthetically known compound, 4-((E)-3,3-dimethylpenta-1,4-dienyl)-phenol (2), and sixteen known natural products, antidesmol (3), mellein (4), sitosterol (5), stigmasterol (6), 5-cholesten-3beta-ol (7), 4-cholesten-3-one (8), 3-(1,1-dimethylallyl)-scopoletin (9), (-)- 5,7-dihydroxy-2-eicosyl-chromone (10), 2,5-dimethoxy-1,4-benzoquinone (11), barbatumol A (12), N-trans-feruloyltyramine (13), syringic aldehyde (14), p-hydroxybenzoic acid (15), taxifolin (16), (+)-catechin (17) and gallocatechin (18), were isolated from the roots of Antidesma acidum. The structural assignment of the new compound was based on spectroscopic methods. All isolated compounds were evaluated for their cytotoxicity (KB and HeLa cells). Compound 1 showed good cytotoxicity against both cell lines (with IC50 values of 2.60 and 3.80 microg/mL, respectively).