Cellulose hydrogels are “green” reductants and supports for Au nanoparticles (Au NPs). Herein, we report a facile method for synthesizing Au NPs based on alkali/urea regenerated cellulose hydrogels. By controlling the precursor concentration and the reaction temperature, the size of the Au NPs can be adjusted. We successfully demonstrate that the Au NP/cellulose hydrogels can be used as efficient heterogeneous catalysts in the reduction of 4-nitrophenol (4-NP). Au NPs with smaller sizes have higher catalytic activity. The highest turnover frequency (TOF) of the Au NPs/cellulose hydrogels is 19.4 h−1. The Au NP/cellulose hydrogels can be easily isolated after the catalytic reaction.