In this work, we presented an environmentally friendly approach for the immobilization of high-density silver nanoparticles (Ag NPs) on the negatively charged surface of alginate (AL) microspheres. Quaternized cellulose (QC)–Ag nanocomposites are subsequently deposited onto the surface of AL microspheres through the electrostatic interaction between QC and AL. The density of Ag NPs immobilized on the surface of AL microspheres could be altered by varying the Ag content in QC–Ag nanocomposites. Optical microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, thermogravimetric analysis, transmission electron microscopy and scanning electron microscopy were employed to follow all preparation steps and to characterize the resulting functional surfaces. The catalytic activity of the composite microspheres was evaluated by reduction of p-nitrophenol to p-aminophenol by NaBH₄. The results demonstrated that the obtained microspheres exhibited excellent catalytic activity with a high reaction constant of 2.75 min⁻¹. This was a significant enhancement compared to other Ag catalysts reported in the literature.