Antibacterial alginate fibers containing silver nanoparticles (AgNPs) have been prepared in this work. Firstly, AgNPs colloid solution was obtained employing alginate as stabilizing agent and glucose as reducing agent. Then, sodium alginate powder was directly added to the above solution and the resultant sodium alginate–AgNPs spinning dope was spun to prepare alginate–AgNPs composite fibers. The fibers were characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction and thermal gravimetric analysis (TG). Mechanical properties of the fibers were also measured. The antimicrobial properties were investigated by Staphylococcus aureus and Escherichia coli, and the results indicated that the composite fibers showed strong antibacterial activity.