The in vitro antimicrobial activity of the marine green algae Chaetomorpha aerea was investigated against gram-positive bacteria, gram-negative bacteria, and a fungus. The water-soluble extract of algae was composed of a sulfated (6.3%) galactan with a molecular weight of 1.160x106 Da and a global composition close to commercial polysaccharides as dextran sulfate or fucoidan. The polysaccharide was composed of 18% arabinose, 24% glucose, 58% galactose. The re-suspended extracts (methanol, water) exhibited selective antibacterial activities against three gram-positive bacteria including Staphylococcus aureus (ATCC 25923). Minimum inhibitory concentration and minimum bactericidal concentration tests showed that the sulfated galactan could be a bactericidal agent for this strain (40mg.mL-1). Results of the present study confirmed the potential use of the green algae Chaetomorpha aerea as a source of antibacterial compounds or active known molecules.