Extracts from 26 marine macroalgal species (11 Phaeophyceae, 7 Chlorophyta, and 8 Rhodophyta) sampled from the lagoons of Tahiti, Moorea, and Tubuai (French Polynesia) were tested for several biological activities. The red macroalga Amansia rhodantha exhibited the strongest antioxidant activities using four complementary methodologies (total phenolic content, 2,2-diphenyl-1-picrylhydrazyl, ferric reducing antioxidant power assay, and oxygen radical absorbance capacity assay). Therefore, the major metabolites of A. rhodantha were isolated and their structures identified. Some brown algae, especially species of the family Dictyotaceae like Padina boryana and Dictyota hamifera, showed cytotoxic activities against murine melanoma cells. Caulerpa chemnitzia extract demonstrated also a strong alpha-glucosidase inhibition (83.8% at 10 mu g mL(-1)) and Asparagopsis taxiformis extract a high acetylcholinesterase inhibition (71.3% at 100 mu g mL(-1)). Lastly, several Polynesian seaweeds demonstrated quorum-sensing inhibition for Vibrio harveyi. These results suggested that some seaweeds from French Polynesia have a great biotechnological potential for future applications in aquaculture, health, or cosmetic industries.