Noble metal nanocatalysts are one of the most promising candidates for various catalysis applications. However, their high surface energy undesirably causes serious stability problems, which remarkably decreases the intrinsic catalytic activity. In this paper, for the first time, we describe the successful fabrication of silver nanoparticles (Ag NPs) in situ grown on magnetically separable alginate-based biohydrogels (Ag@AMH) by an environmentally friendly light-driven approach. In the presence of alginate biopolymers, silver ions can be readily adsorbed and subsequently photoreduced to metallic Ag NPs on the biohydrogels. These Ag@AMH catalysts were characterized by X-ray diffraction (XRD), scanning electronic microscopy (SEM), X-ray energy dispersive spectroscopy (EDS), Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS) and vibrating sample magnetometer (VSM) techniques. The resulting Ag@AMH exhibited excellent and durable activity for the catalytic reduction of 4-nitrophenol to 4-aminophenol by NaBH₄ in aqueous solution, which can be recycled for three successive cycles of the reaction with a conversion efficiency of more than 99%. Such Ag@AMH were thus expected to have the potential as a highly efficient, cost-effective and eco-friendly heterogeneous catalyst for industrial applications.