A free-standing, antibacterial hydrogel was fabricated using silver-nanoparticle-immobilized cellulose nanofibers (CNFs) and alginate. Surface hydroxyl groups of CNFs were oxidized to carboxylate groups using (2,2,6,6-tetramethylpiperidin-1-yl)oxidanyl (TCNF), followed by the treatment with silver nitrate solution for surface adsorption of silver ions. In situ reduction of silver ions to produce silver nanoparticles was performed for the silver-adsorbed CNFs. Electron microscopy, X-ray diffraction, and spectroscopic analysis revealed that higher amounts of silver nanoparticles were immobilized on the surface of TCNF than on the surface of native CNF. Silver-nanoparticle-immobilized TCNF was embedded in alginate gels and silver ions from the matrix were slowly released for 7 days. Silver-nanoparticle-loaded alginate gels showed comparable antibacterial activity to silver-ions-loaded alginate gels, although the former showed a significantly lower cytotoxicity against animal cells. Thus, the antibacterial gels can potentially be applied to various skin surfaces to prevent bacterial infection while minimizing skin damage.