An antioxidant substance (AOS) obtained from an enzymatic extract of the red seaweed Gloiopeltis tenax was purified by DEAE-Sephadex CL-6B and Sephadex G-100 column chromatography. The purification yield of AOS was 14.3%. The AOS predominantly contained fucose, mannose, and galactose but also contained a sulfate group. The structure of AOS was investigated by periodate oxidation, desulfation, Fourier transform-infrared spectroscopy, and nuclear magnetic resonance spectroscopy. AOS was mainly composed of alternating units of $eta$-D-Glc($1;{ ightarrow};2$)$alpha$-D-Man($1;{ ightarrow};4$)$eta$-D-Gal($1;{ ightarrow};4$)$alpha$-D-Man($1;{ ightarrow};4$)$eta$-D-Gal $alpha$-D-Man ($1;{ ightarrow};4$) $eta$-D-Glc (or Xyl)- and branched linkage of $alpha$-D-Man($1;{ ightarrow};3$) $alpha$-D-Fuc. In addition, the fucose residues were shown to be 2-O- and 4-O-sulfated and, therefore, were either terminal or 3-linked. The antioxidative activity of AOS was measured using the 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging assay and the $eta$-carotene.linoleate assay systems and was compared with those of butylated hydroxytoluene and ascorbic acid (AscA). The results showed that AOS exhibited higher antioxidative activity than AscA in the DPPH assay model and in the $eta$-carotene.linoleate assay system at all of the four concentration levels tested (from 50 to $200;{mu}g/mL$). These results suggested that AOS from the red seaweed G. tenax is an efficient novel antioxidant.