The equilibrium and thermodynamics of the biosorption of Pb(II), Zn(II), Cu(II), and Cd(II) onto activated carbon prepared from olive branches were studied under different parameters of pH, initial concentration, and temperature. The batch biosorption procedure was used to find the optimum conditions. The biosorption of each metal ion was found to be pH-dependent. The maximum metal ion biosorption was achieved at pH value 5 for Pb, Cu, and Cd ions and at pH 3 for Zn ions. The extent of the metal ion biosorption increased with temperature (indicating the endothermic character) and initial metal ion concentration. The experimental data of metal ion biosorption were analyzed by Freundlich and Langmuir isotherm models. For all metal ions, the Freundlich isotherm model gave a better fit with higher correlation (R2) to equilibrium data than Langmuir model. The adsorption capacity values were 41.32, 34.97, 43.10, and 38.17 (mg/g) for Pb(II), Zn(II), Cu(II) and Cd(II), respectively. Thermodynamically parameters, like Gibbs free energy (ΔG˚), enthalpy (ΔH˚), and entropy (ΔS˚) were calculated. The biosorption of each metal ion was non-spontaneous and the order of non-spontaneity of the biosorption process being Zn(II) > Cu(II) > Cd(II) > Pb(II). Likewise, change in entropy was noticed for each metal ion and the order of disorder was Pb(II) > Cd(II) > Cu(II) > Zn(II).