The pollution of water resources has become a worldwide concern because of the indiscriminate disposal of heavy metal ions and toxic organics in the past few decades. In this study, we use a sustainable, low cost, and abundant plant protein, soy protein isolate (SPI) as a matrix, polyethyleneimine (PEI) as a functional component to prepare the SPI/PEI composite hydrogels through a facile chemical crosslinking method. The results show that the SPI/PEI composite hydrogels can effectively adsorb Cu(II) ions from aqueous solution. In particular, the SPI/PEI composite hydrogel with a 50% PEI content demonstrates an excellent selectivity for the removal of Cu(II) ions when co-existing competitive heavy metal ions, including Zn(II), Cd(II), and Pb(II), especially the selectivity coefficient for Cu(II)/Zn(II), reaches about 250. Furthermore, the adsorbed Cu(II) ions in the composite hydrogel can be reduced easily in situ by NaBH₄ to form uniformly dispersed copper nanoparticles (Cu NPs), which gives a Cu NP-loaded-SPI/PEI composite hydrogel. Such a material is found to be able to act as a catalyst to catalyse the model reaction, i.e., the reduction of 4-nitrophenol with a high efficiency. These results suggest that such a SPI-based hydrogel is a good candidate to selectively adsorb and recycle copper element for the waste disposal industry. In addition, we also provide a strategy to keep the natural resources sustainable, i.e., to select a natural and sustainable material (SPI) to protect the environment (wastewater treatment), and to recycle metals (copper and possibly others).