Twenty-seven diterpenes of six chemical classes, including seven new diterpenes (1, 2, 6, 10, 11, 16, and 19), have been isolated from a collection of the brown alga Dictyota plectens from the South China Sea. The structures of the new diterpenes were elucidated by extensive spectroscopic analysis and by comparison with reported data. In the in vitro assays, 9, 12, 14, 16, and 22 showed inhibitory activity against HIV-1 replication with IC50 values of 16.1-30.5 μM, compounds 5, 13, 24, and 26 exhibited anti-H5N1 activity with inhibition rates of 50%-62% at 30.0 μM, and 12 and 24 also showed potent inhibition against LPS-induced NO production with inhibition rates of 90% and 86%, respectively, at 10.0 μM.