Swollen double networks (DNs) are hydrogels with greatly improved stiffness, toughness, and strength compared to classical hydrogels. The highest stiffness is achieved for organic− inorganic DN hydrogels, which show a rather low tensile strength of about 1 MPa so far. It was presumed that this is due to an insufficient reversible bond formation between inorganic and organic phases. Therefore, the influence of the functional groups that form reversible bonds between these two phases was investigated on the example of calcium phosphate-based DN hydrogels. The functional groups were introduced by copolymerization of acrylate-based monomers with acrylamide and N,Ndimethylacrylamide, respectively, to form a hydrogel-containing alkaline phosphatase. After enzyme-induced mineralization, it was found that only acrylic acid (AA) results in improved strength of the formed ultrastiff DN hydrogels. Stress−strain curves with different strain rates revealed that Young's modulus of ∼300 MPa is constant in all cases, while the tensile strength increases from 7 MPa at 5% min −1 to 17 MPa at 750% min −1 . The fracture toughness of these optically transparent DN hydrogels, which are among the stiffest and strongest existing hydrogels, is up to 2000 J m −2 which is also improved by the introduction of AA into the hydrogel.