The invention relates to a product resulting from the grafting, by esterification or transesterification, onto at least a part of the hydroxyl functions of an ulvan-type polysaccharide in the form of an acid or in the form of a mono- or divalent salt, in particular a sodium salt, of fatty chains or of mixtures of fatty chains containing 8 to 28 carbon atoms, said fatty chains being saturated or unsaturated, and linear or branched. It also relates to the method for preparing this product. It also relates to uses of this product, in particular as surfactant.