Macroalgae produce a large range of primary and secondary metabolites with ecological and economical importance. Studies on antioxidants from marine algae have increased notably, focusing on searching new sources of natural compounds for different applications, in which tropical species have been shown to have high potential, often improved by stressful environmental conditions during the tide cycle with periods of emersion and submersion. Therefore, in order to characterize the antioxidant activity and relate it to local environmental tide exposure, three species of brown marine algae,Canistrocarpus cervicornis,Dictyopteris delicatula, andLobophora variegatafrom two beaches on the northeast coast of Brazil were studied. Dichloromethane:methanol (DCM:M) and aqueous extracts were tested for ferric reducing antioxidant power, 2,2-diphenyl-1-picrylhydrazyl, and 2,2 '-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) antioxidant assays and total phenolic compounds. Aqueous extracts ofC. cervicornisshowed up to 10 times major antioxidant activity and phenolic compounds than DCM:M extracts. Different characteristic of antioxidant activity were identified among the beaches, species, and extracts, in which aqueous extracts fromC. cervicornisandL. variegatashowed the most promising matrices for future prospection of natural antioxidants.