We present here a straightforward, one-step “green” approach for preparing Pd nanoparticles of controlled size and size distribution. The new catalysts were synthesized using a low-cost, biocompatible cellulose, sodium carboxymethyl cellulose (CMC), as a stabilizer and ascorbic acid as a reducing agent at temperatures ranging from 22 to 95 °C. The mean size and polydispersivity (expressed as standard deviation, SD) of the Pd nanoparticles was exponentially reduced by increasing the preparation temperature from 22 to 95 °C. At 95 °C, nearly monodisperse Pd nanoparticles were obtained with a mean diameter of 3.6 nm (SD = 0.5 nm). The Pd nanoparticles exhibited high catalytic reactivity when tested for hydrodechlorination of trichloroethene in the presence of H₂. The observed pseudofirst-order reaction rate constant, k_obs, was up to 692 L g⁻¹ min⁻¹, which is comparable to the Pd nanoparticles synthesized per the conventional borohydride reduction method. This new approach not only offers a simple way to manipulate particle size and size distribution but also eliminates the need of borohydride, which is much more costly and less environmentally friendly than the ascorbic acid used in this work.