Twelve new meroditerpenoids, 1-12, along with eight known compounds, have been isolated from the brown alga Cystoseira usneoides collected off the coast of Tarifa (Spain). The structures of the new metabolites have been established by spectroscopic techniques. All of the new compounds consist of a toluhydroquinone-derived nucleus linked to a regular diterpenoid moiety, which can either be acyclic or contain an ether ring. Most structural diversity arises from the presence of different oxygenated functionalities and unsaturations along the two terminal isoprenoid units of the diterpene backbone. Twelve of the isolated meroditerpenes have been tested in antioxidant assays. All of them have shown radical-scavenging activity. The most active compounds were cystodiones G (1) and H (2), 11-hydroxyamentadione (15), and amentadione (16), which exhibited antioxidant activities in the range of 77-87% that of the Trolox standard. In anti-inflammatory assays, cystodiones G (1) and M (6), cystone C (9), 11-hydroxyamentadione (15), and amentadione (16) showed significant activity as inhibitors of the production of the proinflammatory cytokine TNF-α in LPS-stimulated THP-1 human macrophages.