A micro algal growth-enhancing polysaccharide compound was isolated from the green alga Monostroma nitidum using water extraction, molecular fractionation, a DEAE-cellulose column, and fast protein liquid chromatography using a Superose-12 column. The yield of the compound from the seaweed powder was 8.3$ imes$l0$^{-3}$%. At 2 mg/mL concentration, the polysaccharide enhanced Tetraselmis suecica cell growth in f/2 medium by approximately 160%.