Edible seaweeds are usually sold as flakes or even powers, and morphological identification of seaweed taxa is difficult. Water-soluble polysaccharides (WSPs) in edible seaweeds are not only interesting functional food ingredients (e.g. gel-forming property, health benefits), but also useful for seaweed taxon differentiation. The current study aims to explore the utility of monosaccharide profiling of WSPs in seaweed differentiation. We developed a high-performance liquid chromatography-photodiode array detection for monosaccharide determination, and characterized monosaccharide profiles of WSPs from five edible seaweeds sold in Iceland (i.e. kombu Iceland – Laminaria hyperborea, sugar kelp – Saccharina latissima, dulse – Palmaria palmata and two types of nori labelled as Porphyra sp. and Pyropia sp.). Monosaccharide profiling data reflected both the complexity and quantitative differences of WSPs. The seaweed dulse showed the highest concentrations of total sugar (ca. 16 mM), followed by kombu (ca. 12 mM). From monosaccharide profiling of red algae, it was found that galactose (ca. 81–87% of total sugars) and xylose (ca. 65–77% of total sugars) are the dominant sugars in nori and dulse, respectively, which reflected the presence of galactans and xylans. In brown algae (i.e. kombu and sugar kelp), glucose (ca. 64–83%) and fucose (13–20%) are the main sugars, and they represent laminarins and fucoidans. The principal component analysis of sugar profiles showed the useful patterns ‘for seaweed taxon differentiation. The dendrogramresulting from hierarchical cluster analysis is congruent with phylogenetic tree, implying the chemotaxonomic value of seaweed monosaccharide profiles. This tool will be more useful in authentication of seaweed taxa when morphological and genetic identifications are not available.