Two new diterpenoids, anthogonoid A and antsimplexin A (1-2), as well as a known diterpenoid, klysimplexin G (3), were isolated from a Beibu Gulf gorgonian coral, Anthogorgia caerulea. The structures of these compounds have been established by detailed spectroscopic analysis and by comparison with spectral data of related known compounds. Compounds 1-3 showed significant antifouling activity against the larval settlement of Balanus amphitrite. Graphical Abstract Two New Diterpenoids from the Beibu Gulf Gorgonian Anthogorgia caerulea