Four new cyclized diterpenes, one xenicane (1) and three dolabellanes (2-4), were isolated, along with seven previously reported metabolites [3beta-hydroxydilophol (5), dictyols E (6) and C (7), hydroxycrenulide (8), 9-acetoxy-15-hydroxy-1,6-dollabelladiene (9), hydroxyacetyldictyolal (10), and fucoxanthin], from a Mediterranean species of Dictyota sp. collected in Le Brusc Lagoon (French Mediterranean coast). Their structures, as well as their relative configurations, were determined through extensive spectrometric (IR, HRESIMS, 1D and 2D NMR) data analysis and molecular modeling studies and by comparison with those reported in literature. Some of the isolated metabolites were evaluated for their antiadhesion activity against a marine bacterial biofilm (Pseudoalteromonas sp. D41).