In the present study we report the first chemical study of the sponge Dictyonella marsilii and the isolation and structure elucidation of the new triterpenoid saponin eryloside W belonging to the eryloside family. A new saponin derivative named eryloside W was isolated from the marine sponge Dictyonella marsilii (Demospongiae, Halichondrida, Dictyonellidae). The structure of the compound was elucidated through extensive use of 1D and 2D nuclear magnetic resonance and mass spectrometry. Determination of the absolute configuration was performed using electronic circular dichroism (ECD) by comparison of experimental and time dependent density functional theory (TDDFT) calculated spectra. This is the first occurrence of a saponin derivative produced by a marine sponge present in the Mediterranean.