Growth, biochemical composition and antioxidant capacity of Rhodymenia pseudopalmata were studied under controlled laboratory conditions. Cultures were exposed to five salinities ranging from 20, 25, 30, 35 and 40 psu under three photosynthetic active radiations (PAR) 50, 100 and 200 μmol photons m-2s-1. Rhodymenia pseudopalmata daily growth rates were significantly higher at 20 psu and high PAR, and decreased at 40 psu and low PAR. Chl a and PE contents were lower at high PAR. The carbohydrate content increased up to 54 % DW-1 under high PAR and 20 psu. Protein ranged from 4.9 to 10.5 % DW-1. Sixteen amino acids were detected in R. pseudopalmata under the experimental conditions tested. Glutamic acid, proline and methionine may act as osmolytes, while histidine, methionine and tyrosine may be involved in R. pseudopalmata antioxidant capacity. Rhodymenia pseudopalmata grown under high PAR and 20 psu had the highest increase in total phenolic compounds. Moreover, DPPH radical scavenging activity increased at 20, 25 and 40 psu under low PAR, and at 20 psu under high PAR. These results may partly explain the ability of R. pseudopalmata to grow under extreme conditions at the intertidal and its acclimation for mariculture.