A simple, novel method was developed for synthesizing cellulose (CE) fibers doped with silver nanoparticles (Ag NPs) in the green solvent tetrabutylammonium hydroxide / dimethyl sulfoxide / H2O at room temperature. Tetrabutylammonium hydroxide accelerated the reduction of Ag+ to Ag by the cellulose chains, yielding Ag NPs in cellulose solution stabilized using polyethyleneimine (PEI). After 24 h, almost all the Ag+ was reduced to Ag NPs. The influences of silver nitrate concentration, reaction time, and stabilizer on the formation of Ag NPs were investigated by UV-vis spectrophotometry. The prepared smooth and dense cellulose / Ag NP fibers showed high mechanical properties, with a tensile strength of 304.3 MPa and an elongation at break of 22.1%. The fibers exhibited excellent antibacterial activities against Escherichia coli and Staphylococcus aureus, with more than 99% of E. coli bacteria killed by Ag NP / cellulose fibers. The synthesis procedure offers a general and mild approach to designing materials of almost any shape.