Two naturally occurring diterpenes featuring unprecedented tetracyclic skeletons, ioniols I and II (1 and 2, resp.), along with two previously reported metabolites 3 and 4, were isolated from the organic extract of Sphaerococcus coronopifolius collected from the rocky coasts of Corfu island in the Ionian Sea. The structures of the new natural products, as well as their relative configuration, were elucidated on the basis of extensive spectral analysis, including 2D-NMR experiments. The isolated metabolites were evaluated for their antibacterial activity against a panel of Staphylococcus aureus strains, which included multidrug-resistant (MDR) and methicillin-resistant Staphylococcus aureus (MRSA) strains.