Eco-friendly synthesis of metal nanoparticles using plant extracts plays a vital role in green nanotechnology. In this paper, a green method for synthesizing silver nanoparticles (AgNPs) is described by treating silver ions with the aqueous bark extract of Acanthopanax sessiliflorus at ambient temperature. The synthesized AgNPs were characterized by UV–Vis spectrophotometry, X-ray diffractometry (XRD), Fourier transform infrared spectroscopy and transmission electron microscopy (TEM). The formation of AgNPs was monitored using a UV–Vis spectrophotometer, which exhibited a maximum absorption peak (λmax) at 435 nm, and also a color change from light yellowish to yellowish-brown was observed. Powder XRD analysis revealed the formation of fcc crystals, and TEM analysis revealed formation of roughly-spherical shaped AgNPs with an average diameter of ~20 nm. Thermogravimetric analysis showed a two-step thermal decomposition of AgNPs, which majorly started at 150 °C and continued gradually until 700 °C. These AgNPs were found to catalyze the reduction of p-nitroaniline to p-phenylenediamine in the presence of sodium borohydride (NaBH4). The catalytic reduction reaction followed pseudo-first order kinetics with a reaction rate constant (k) of 8.7 × 10−3 min−1. In vitro cytotoxic assay revealed dose-dependent toxicity of these AgNPs in human keratinocytes at higher concentrations. The described green synthesis of these catalytic AgNPs is cost-effective, sustainable, and compatible for industrial and biomedical applications.