Novel clay-polymer composite hydrogels with high water content (up to 98 wt %) are developed, in which mechanical properties are reinforced by the formation of multiple ion-pairs between the polymer chains and clay nanosheets (CNS). When a small amount of guanidinium-pendant methacrylamide (0.1-0.2 wt %) is copolymerized with a neutral monomer (0.5-2.0 wt %) in an aqueous dispersion of CNS (1.0-3.0 wt %), a self-standing hydrogel with satisfactory mechanical toughness and elasticity results, despite its high water content (95-98 wt %). The mechanical properties and swelling behaviors of the hydrogels can be tuned by the amount of the guanidinium-pendant acrylamide. A systematic study indicates that the ion pairs, formed between the guanidinium groups in the polymer chains and the oxyanions on the surfaces of the CNS, serve as crosslinking points in the three-dimensional network developed in these hydrogels. (c) 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014, 52, 839-847