A series of silica-gel-supported sulfur-capped PAMAM dendrimers (SiO2-G0-MITCSiO2-G2.0-MITC) were synthesized and used for the adsorption of Hg(II) from aqueous solution. The optimum adsorption pH was found to be 6. Adsorption kinetics indicated that equilibrium can be approached in about 220 min and that the adsorption capacity increased with increasing generation of sulfur-capped PAMAM dendrimers. The kinetics of the adsorption process was found to be controlled by film diffusion and to follow a pseudo-second-order model. The adsorption isotherms were fitted well by the Langmuir isotherm model, and adsorption was found to take place by a chemical mechanism. Thermodynamic analysis demonstrated that the adsorption was a spontaneous, endothermic, and randomness-increasing process. Adsorption selectivity experiments showed that SiO2-G0-MITCSiO2-G2.0-MITC can selectively adsorb Hg(II) from binary systems containing Hg(II) with Ni(II), Cd(II), Fe(III), and Zn(II). DFT calculations revealed that G0-MITC interacts with Hg(II) through the S atom in a monocoordinated manner, whereas G1.0-MITC behaves as a pentadentate ligand to coordinate with Hg(II) through the N atom of the tertiary amine group, the O atoms of the amide groups, and the S atoms. Charge transfer from G0-MITC and G1.0-MITC to Hg(II) was found to occur during the adsorption process.