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Highlights • Gracilariopsis hommersandii is a potential source of agar for industrial applications. • The gel strength of native and alkali-modified hot water extracts was determined. • Carrageenan diads were found for the first time in the Gracilariales. • Sulfated galactans have unusual sulfation pattern with apiuronic acid side chains. Abstract Red seaweed Gracilariopsis hommersandii produces important amounts of non-gelling galactans, which were extracted with hot water (GrC, yield, 37%, viscosity average molecular weight, Mv 109 kDa), comprising agarose and sulfated galactan structures. The alkali modified derivative, GrCTr (Mv 95 kDa), gave a galactose:3,6-anhydrogalactose molar ratio of 1.0:0.9, and a more regular structure, favouring gelation (melting and gelling temperatures 64 and 14 °C, respectively). The rheological properties of this product suggest possible applications as hydrocolloid. G. hommersandii also biosynthesizes non gelling sulfated galactan fractions with diads constituted by β- d- galactose and partially cyclized α- l -galactose units or non-cyclized α- d- galactose residues. Sulfation was mainly detected on C6 or C4 of the β- d- galactose units, and on C6 and, in minor amounts, on C3 of the α- l -galactose units. The presence of β-apiuronic acid was demonstrated for these fractions as side chains of the galactan backbone. Carrageenan structures were found for the first time in an agarophyte of the Gracilariales. [ABSTRACT FROM AUTHOR]