Integrating high stiffness, strength, and toughness on par with those of soft tissues into synthetic hydrogels is extremely challenging. We have proposed a method to overcome this problem: in situ polymerization of a polymer matrix in layered cellulose nanofibers. In an attempt, ionically cross-linked poly(acrylamide-co-acrylic acid) is fabricated in a wet cellulose nanofiber cake. The resulting hydrogels, called ionically cross-linked nanocomposite (ICN) hydrogels, exhibit a readily adjustable elastic modulus (11.9–190.0 MPa) and high fracture strength (generally > 10 MPa), which are comparable with those of skin and ligament. The high frictional force between the cellulose nanofibers and matrix is responsible for the stiffness of ICN hydrogels; while the tough matrix and weak direct interfibrillar interactions enable good stretchability. We expect that various kinds of cellulose nanofiber/polymer nanocomposite hydrogels with excellent mechanical properties and/or other features can be fabricated by simply changing the monomers.