We carboxymethylated ulvan, extracted from the green algae Ulva lactuca, and chitosan, extracted fromLoligo forbesis squid-pen, yielding polysaccharides with an average degree of substitution of ∼98% (carboxymethyl ulvan, CMU) and ∼87% (carboxymethyl chitosan, N,O-CMC). The carboxymethylation was confirmed by FTIR and quantified by conductimetric titration and 1H NMR. The average molecular weight increased with the carboxymethylation (chitosan: Mn 145→296 kDa and Mw 227→416 kDa; ulvan: Mn139→261 kDa and Mw 368→640 kDa) indicating successful chemical modifications. Mixtures of the modified polysaccharides were tested in the formulation of polyacrylic acid-free glass-ionomer bone cements. Mechanical and in vitro bioactivity tests indicate that the inclusion of CMU in the cement formulation, i.e. 0.50:0.50 N,O-CMC:CMU, enhances its mechanical performance (compressive strength of 52.4 ± 8.0 MPa and modulus of 2.3 ± 0.3 GPa), generates non-cytotoxic cements and induces the diffusion of Ca and/or P-based moieties from the surface to the bulk of the cements.