Five new spongian diterpenoids 1−5 have been isolated from the mollusk Glossodoris atromarginata along with four known related metabolites (6−9). The structures were solved by extensive spectroscopic analysis and chemical correlation. The absolute stereochemistry of compound 2 was determined by Mosher's method. A dietary origin is inferred for all these molecules on the basis of their structural analogies with known sponge metabolites, but their transfer from the digestive glands to the dorsal skin suggests a defensive role to thwart predation.