The temperature- and pH-sensitive poly(N-isopropylacrylamide-co-acrylic acid) hydrogels, poly(NIPAM-co-AA), were synthesized by radical polymerization. The characterizations of hydrogels based on N-isopropylacrylamide (NIPAM) and acrylic acid (AA) before and after adsorption of heavy metal ions was performed by Fourier transform infrared spectroscopy and scanning electron microscopy. Heavy metal ions (Cr, Mn, Pb) adsorbed onto poly(NIPAM-co-AA) hydrogels were identified using the energy-dispersive X-ray spectroscopy. The mechanism of the water transport within the matrix of synthesized poly(NIPAM-co-AA) hydrogels at pH 4.5 is Super Case II diffusion, and at pH 6.8 corresponds to the non-Fickian diffusion mechanism. The effect of pH, temperature, contact time, and the initial concentration of heavy metals on the adsorption process of Cr(VI), Mn(II), and Pb(II) ions from aqueous solutions onto poly(NIPAM-co-AA) hydrogels were investigated. The kinetic and equilibrium data were best fitted by the pseudo-second-order model and Langmuir adsorption isotherm. Thermodynamic results indicate that the removal process of heavy metal ions from aqueous solutions by poly(NIPAM-co-AA) hydrogels was spontaneous and exothermic in nature. Maximum adsorption capacities of poly(NIPAM-co-AA) hydrogels for heavy metal ions decrease in the following order: Pb(II)>Cr(VI)>Mn(II).