The shells of Chinese chestnuts (Castanea mollissima) are an agricultural residue. This work aimed to evaluate this feasibility of using steam explosion to modify this residue for Cu(II) biosorption from aqueous solutions. Equilibrium, kinetic and thermodynamic parameters were evaluated. The steam-explosion pretreatment increased the surface area of the chestnut shell and exposed more hydroxyl and carboxyl groups, which are binding sites for Cu(II). It changed the sorption from a spontaneous process driven by enthalpy to a nonspontaneous one driven by entropy. It increased the Cu(II) sorption capacity at higher temperatures while it decreased the capacity at lower ones. Compared with untreated chestnut shell, the steam-exploded shell is preferable for Cu(II) sorption at higher temperatures.