Six new spongian diterpene derivatives, ceylonins A-F (1-6), were isolated from the Indonesian marine sponge Spongia ceylonensis along with spongia-13(16),14-dien-19-oic acid (7). They contained three additional carbons in ring D to supply an ether-bridged bicyclic ring system. Their structures were elucidated by analyzing NMR spectroscopic data and calculated ECD spectra in comparison to experimental ECD spectra. The bicyclic ring system may be derived from the major metabolite 7 and a C3 unit (an acrylic acid equivalent) through an intermolecular Diels-Alder reaction, which was experimentally supported by the formation of 1-6 from 7 and acrylic acid. The inhibitory effects of the isolated compounds on the RANKL-induced formation of multinuclear osteoclasts in RAW264 macrophages were examined.