Alginate, a naturally abundant and environmentally friendly carbohydrate polymer, can form ionically crosslinked hydrogels with Ca2+. Therefore, a novel design of interpenetrating nanocomposite hydrogels (PMACa) is proposed, which is synthesized by in situ polymerization of acrylamide in the presence of montmorillonite platelets and sodium alginate at an elevated temperature, followed by secondary crosslinking of the hydrogel using a CaCl2 solution. PMACa displays superior tensile strength and strain at break under the fully swollen state, as well as rapid shape recovery and insignificant residual strain upon the release of an external load. Furthermore, PMACa also shows remarkable reversible pH-dependent swelling/deswelling behaviors due to the pH-sensitivities of alginate network and montmorillonite platelets. These new high-performance interpenetrating alginate-containing nanocomposite hydrogels may find potential applications as aquatic load-bearing materials for artificial tissues, actuators, agriculture, etc.