Three new diterpenes Amijiol acetate (3), dolabellane, Dolabellatrienol (4), and dolastane, Amijiol-7, 10-diacetate (9) were isolated together with the previously described Pachydictyol A (1), Isopachydictyol A (2), 8β-hydroxypachydictyol A (5), Amijiol (6), Isodictyohemiacetal (7) and Dictyol C (8) from the Red Sea brown alga Dictyota dichotoma var. implexa. The structures and relative stereochemistry of the new diterpenoids were proposed on the basis of their spectral data. Compounds 3 and 9 have potent activity against DNA damage, cytotoxicity against WI-38, HepG2, and MCF-7 cell lines, and antioxidant using ABTS and erythrocytes hemolysis.