The chitosan/poly(vinyl alcohol) (CTS-PVA) hydrogel adsorbent with three-dimensional network structure showed superior adsorption properties for Hg(II) ions. The pH influence study showed Hg(II) adsorption on the hydrogel adsorbent is strongly pH-dependent. The adsorption capacity in pH 2.00 solution (200.20. mg/g) is higher than that in pH 11.00 solution (140.27. mg/g), which indicated that Hg(II) adsorption on the hydrogel adsorbent was not only by electrostatic interactions. FT-IR spectral analysis before and after Hg(II) adsorption represented that the functional groups responsible for Hg(II) adsorption changed with the pH of solutions. Thermodynamic studies revealed that Hg(II) adsorption on the hydrogel adsorbent is a favorable, spontaneous, and endothermic chemisorption process. The binding force between Hg(II) ions and functional groups was strengthened at high temperatures, which made the adsorption more favorable at high temperatures. The adsorption process fitted well by the Langmuir and the Freundlich isotherm models. The Langmuir maximum adsorption capacity increased from 697.70 to 950.62. mg/g with increasing temperature from 20 to 60. °C. This study will contribute to an in-depth understanding of adsorption phenomena, and provide a scientific and engineering basis for the practical application of the CTS-PVA hydrogel adsorbent.