The ever-increasing water pollution by synthetic dyes, heavy metals, and other organic contaminants are critical environmental issues that receive the world attention. Considering the purification technology based on adsorption, the fabrication of efficient adsorbents for the effective separation and removal of water contaminants is urgently required. Here, we developed a polyanionic superabsorbent hydrogel by graft copolymerization of methacrylic acid, acrylamide, and 2-acrylamido-2-methyl-1-propanesulfonic acid onto sodium alginate. In this study, the capacity of as-prepared hydrogel to remove Cd2+ ions and methylene blue from aqueous solutions was investigated using batch equilibrium experiments. In this regard, the effect of different contact times, pH, sorbent dosage, contaminant concentrations, temperature, and salinity on the hydrogel adsorption capacity was studied. To understand the hydrogel sorption behavior, the adsorption kinetics using pseudo-first-order, intraparticle diffusion, Elovich models, Langmuir, Freundlich, Temkin, and Dubinin-Radushkevich isotherms were employed. The hydrogel achieved remarkable removal efficiencies for methylene blue up to 131.5 mg/g (97.5%) and 526 mg/g for Cd2+ ions (87%). More interestingly, the regeneration experiment validated the reusability of SAH. These findings highlight the potential of SAH hydrogel for the effective removal of Cd2+ ions and methylene blue from water treatment applications.