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A water-soluble sulfated polysaccharide, F2-1, was obtained from the marine green alga, <i>Codium isthmocladum</i>, using ion-exchange and size-exclusion chromatography. Structure analysis showed that the F2-1 was a sulfated arabinan comprising Ara, Rha, Man, Gal, and Xyl with an 18% sulfate content and a molecular weight of 100 kDa. Methylation analysis combined with desulfation, GC-MS, IR, and NMR spectroscopy showed that the backbone of F2-1 was →4)-β-L-Ara<i>p</i>(1→ residue. Its 2-<i>O</i> and/or 3-<i>O</i> positions showed sulfate modification; additionally, the 2-<i>O</i> or 3-<i>O</i> position showed branch points. The side chains were composed of →5)-β-D-Gal<i>f</i>, (1→2,6)-β-D-Gal<i>f</i>(1→, (1→2)-β-L-Rha<i>p</i>4S, →4)-α-D-Glc<i>p</i>(1→, and terminal α-D-Gal<i>p</i>(1→ and β-D-Xyl<i>p</i>(1→. Polysaccharides containing β-D-galactofuranose are rarely found in seaweed. F2-1 exhibited significant anticoagulant activity in vitro. Our findings suggested that the green-tide alga, <i>Codium isthmocladum</i>, can be considered as a useful resource for bioactive polysaccharides.