A new decenoic acid derivative, gelliodesinic acid (1), and a natural new indole alkaloid (2), together with three known furanoterpenoids (3-5) and two known indole alkaloids (6-7), were isolated from the MeOH extract of the marine sponge Gelliodessp. collected in Vietnam. The chemical structures of 1-7were determined by analyses of 1D-and 2D-NMR and MS data and by comparisons of the data with those reported in the literature. The cytotoxicity assay against HeLa, MCF-7, and A549 cancer cell lines revealed that the known furanoterpenes 3-5exhibited cytotoxic activities, with IC50values ranging from 23.6 to 75.5 μM, against the three cell lines, and that 6showed cytotoxicity with an IC50 value of 89.2 μM against A549 cancer cell line.