A Ru supported on a magnetically separable chitosan-based nanomaterial (Mn@CS@Ru) was prepared by wet impregnation based on ionic gelation using sodium tripolyphosphate as a cross-linking agent. The ionic gelation of chitosan leads to a supporting matrix to promote the embedding of manganese(II) ferrite and Ru nanoparticles (NPs) by electrostatic interactions. The effects of the formulation and method parameters on the fabrication process were investigated, and the resulting as-prepared Mn@CS@Ru nanocatalyst was characterized. The catalytic activity of the Mn@CS@Ru nanomaterial was evaluated in the reduction of 4-nitrophenol (4-NP) and 4-nitroaniline (4-NA) in the presence of sodium borohydride as a reducing agent at room temperature. The turnover frequency values in the reduction of 4-NP and 4-NA were 273.9 and 336.5 min−1, respectively, which were attributed to the very small size of the hybrid nanomaterial (32.0±2.8 nm with 3.9±0.1 nm Ru NPs) that provided a large surface-area-to-volume ratio for the chemical reaction. Furthermore, the hybrid nanocatalyst was recovered easily by magnetic separation after the catalytic reaction and could be reused in at least 10 cycles without a loss of catalytic activity, which confirms its high stability. The present route is a new approach to synthesize highly active magnetic heterogeneous catalysts for the reduction of nitroarenes based on metallic NPs with easy accessibility, excellent activity, and convenient recovery.