In this study, we developed a new approach in the preparation of chitosan-based polymeric nitric oxides. Chitosan film (unreacted chitosan) reacted with glutaraldehyde to introduce aldehyde groups onto the material surface (glutaraldehyde-treated chitosan). Glutaraldehyde-treated chitosan reacted with a small-molecule nitric oxide donor, 3,3-bis(aminoethyl)-1-hydroxy-2-oxo-1-triazene, to covalently immobilize nitric oxide–releasing moieties onto the polymer (chitosan-based polymeric nitric oxide). Chitosan-based polymeric nitric oxide showed sustained release of nitric oxide. The activation energies and rate constants of nitric oxide release were determined. The released nitric oxide provided potent antimicrobial effects against Gram-positive and Gram-negative bacteria living in biofilms, and the chitosan-based polymeric nitric oxide film showed added/synergistic effects with common antibiotics. At 4°C, the chitosan-based polymeric nitric oxide could be stored for more than 1 month, without significantly losing nitric oxide–releasing capabilities. Furthermore, chitosan-based polymeric nitric oxide showed excellent biocompatibility with mammalian cells, pointing to great potentials of the new materials for a wide range of biomedical applications.