Cellulose (Cel) nanofibrils (CNFs) produced by periodate oxidation of native cellulose fibers were functionalized with silver (Ag) nanoparticles (NPs) using Tollens' reaction. The morphology and chemical composition of the resulting Cel-Aghybrid nanofibrils were characterized using transmission electron microscopy (TEM), X-ray diffraction (XRD), as well as Fourier-transform infrared (FTIR) and UV–Vis spectroscopies. To check whether the hybridization with Ag affected the reinforcing potential of the CNFs, nanocomposite films based on an acrylic matrix filled with the as-prepared Cel-Ag nanofibrils at different contents were processed by film casting. Their mechanical properties were investigated by dynamic thermomechanical analysis (DMTA). The hybrid Cel-Ag nanofibrils exhibited good bactericidal properties against Gram+ and Gram− bacteria. Interestingly, the presence of Ag NPs did not seem to affect the reinforcing potential of the nanocellulose, and the amount of Ag leached out from films was below the permissible limit of 12 ppb. Nanocomposites based on this hybrid Cel-Ag nanofiller thus have a great potential in the field of active packaging films, coating and adhesives with enhanced antibacterial activity.