Three polyamine-functionalized macroporous polystyrene-co-divinylbenzene beads prepared by ethylenediamine (EDA), diethylenetriamine (DETA), and triethylenetetramine (TETA) modification, respectively (called PS-FDA, PS-DETA, and PS-TETA), have been investigated as high-capacity chelating resins for Hg(II) ions. Their structures were characterized by elemental analysis, infrared spectroscopy, and porous analysis. PS-EDA, PS-DETA, and PS-TETA were found to be highly effective for the adsorption of Hg(II) ions, exhibiting uptake capacities of (1.55, 1.73, and 1.84) mmol of Hg(II) ions/g of adsorbent, respectively. Kinetic data indicated that the adsorption process achieved equilibrium within 10 h and followed a pseudo-second-order rate equation. The adsorption isotherm data fit the Frendlich model and its linearized form well, together with thermodynamic data indicating the spontaneous and endothermic nature of the process. Results of a desorption study showed that Hg(II) ions adsorbed onto PS-FDA, PS-DETA, and PS-TETA could be easily desorbed. PS-FDA, PS-DETA, and PS-TETA show excellent affinity for Hg(II) ions, removing them from mixed metal solutions at pH 2.0. The highest effectiveness of PS-TETA for the capture of Hg(II) ions is attributed to both the longer polyamine chain and the expansion capability of the polystyrene-co-divinylbenzene framework, which facilitates the accessibility of the binding sites.