Three new briaran diterpenes, 2β-acetoxy-2-(debutyryloxy)stecholide E (3), 9-deacetylstylatulide lactone (4), and 4β-acetoxy-9-deacetylstylatulide lactone (5), and two known diterpenes, brianthein W (1) and 9-deacetylbriareolide H (2), have been isolated from a Briareum sp. of gorgonian. The structures and relative stereochemistry of these compounds were determined by spectroscopic and chemical methods. Diterpenes 2−4 and the corresponding 9-acetyl derivatives 6, 7, and 12 exhibited cytotoxicity toward various cancer cell lines.