We report the isolation, for the first time by phage display, of a scFv recombinant antibody called B3 directed against carrageenans, the major sulphated polysaccharides of red seaweeds. Immunoassays were used to characterize the binding of B3 antibodies toward the three main carrageenan forms (ι, κ, and λ) differing by their sulfonic ester content and the presence of 3,6-anhydrogalactose. In enzyme-linked immunoadsorbent assay (ELISA), B3 soluble scFv showed a high reactivity towards ι-carrageenan at any titer but, at high titer only, recognized also the highly sulfated λ-form. Surface-adsorbed κ-polymers were only recognized in presence of poly-L-lysine (PLL). The replacement of Na ions by K in the buffers had no effect on κ-polymer detection but increased the binding of B3 antibodies toward both ι- and λ-carrageenans, whereas addition of Ca decreased sharply the recognition of the ι-form. In competitive assays, low titer B3 soluble scFv showed a ι>κ>λ selectivity and recognized a mixture of ι-oligomers with degrees of polymerization between 4 and 18 but not sub-fractions of 4 or 6 residues long. We suggest therefore that the B3 epitope could consist of a helical conformation of carrageenan chains. Immunofluorescence microscopy showed that, amongst other red algae, Chondrus gametophyte (containing ι-chains) was strongly recognized by B3 scFv whereas sporophytic tissues rich in λ-carrageenans were not, assessing the preference of this probe for ι-carrageenans in situ. The high potential of the B3 recombinant probe is discussed.