A new decenoic acid derivative, gelliodesinic acid, and a naturally new alkaloid, together with three known furanoterpenoids and two known indole alkaloids, were isolated from the MeOH extract of the marine sponge Gelliodes sp. collected in Vietnam. The chemical structures of the isolated compounds were 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 three known furanoterpenes exhibited cytotoxic activities with IC(50) values ranging from 23.6 to 75.5 muM against the three cell lines, and that 1H-indole-3-carboxylic acid showed cytotoxicity with an IC(50) value of 89.2 muM against A549 cancer cell lines. CI - (c) 2020 Wiley-VHCA AG, Zurich, Switzerland.