Abstract: The invention concerns a method for preparing a modified biodegradable polymer in aqueous medium comprising at least two steps. The first step is a reaction between an amino acid, a peptide or a polypeptide and maleic anhydride to form a compound having an unsaturated vinyl-carboxylic acid function. In the second reaction step, the unsaturated diacid obtained in the first step is reacted with a biodegradable polymer having at least one primary amine function, such as a fibrous protein or a glycosaminoglycan. The preferred polymer used is collagen or chitosan. The invention also concerns the modified biodegradable polymer obtained by the method. The invention further concerns a biomaterial or a dressing containing the modified biodegradable polymer having biocompatible, cytocompatible, hemostatic, bactericidal and wound healing properties, and its medical, biomedical, pharmaceutical or cosmetic use.