A glycosaminoglycan derivative endowed with heparanase inhibitory activity and antitumor activity, bearing carboxylate groups in positions 2 and 3 of at least part of the glycosaminoglycan residues, and to the process for preparing the same. The glycosaminoglycan derivatives of the present invention are generated starting from natural or synthetic glycosaminoglycans, preferably heparin or low molecular weight heparin, optionally 2-O- and 2-N-desulfated by two steps of oxidation. By the first oxidation, adjacent dials and optionally adjacent OH/NH2 of the glycosaminoglycan residues are converted to aldehydes and by the second oxidation said dialdehydes are converted to carboxylate groups. The first oxidation preferably leads to the cleavage of C2-C3 linkage of the ring of oxidable residues. The invention relates to a process for the preparation of said glycosaminoglycan derivatives and to their use as active ingredients of medicaments.