The green algae of the genus Codium have recently been demonstrated to be an important source of sulfated galactans from the marine environment. Here, a sulfated galactan was isolated from the species Codium isthmocladum and its structure was studied by a combination of chemical analyses and NMR spectroscopy. Two fractions (SG 1, ∼14 kDa, and SG 2, ∼20 kDa) were derived from this highly polydisperse and heterogeneous polysaccharide. Both exhibited similar structures in H 1D NMR spectra. The structural features of SG 2 and its desulfated derivative were analyzed by COSY, TOCSY, DEPT-HSQC, HSQC, and HMBC. This sulfated galactan is composed preponderantly of 4-sulfated, 3-linked β-D-galactopyranosyl units. In minor amounts, it is sulfated and glycosylated at C-6. Pyruvate groups are also found, forming five-membered cyclic ketals as 3,4-O-(1ʼcarboxy)-ethylidene-β-D-galactose residues. A comparison of sulfated galactans from different marine taxonomic groups revealed similar backbones of 3-β-D-Galp-1.