It is highly desired yet challenging to develop hydrogels with a combination of excellent mechanical properties and good stability for potential applications. Here we report kappa-carrageenan/polyacrylamide double-network (DN) hydrogels with remarkable mechanical performances and high stability in water, which are achieved using zirconium ions to further cross-link the first physical network of kappa-carrageenan by forming coordination complexes. Thus, obtained DN hydrogels in equilibrated state with water content of 83-91 wt % were highly transparent and showed tensile breaking stress of 1.5-3.2 MPa, breaking strain of 300-2200%, Young's modulus of 0.2-2.2 MPa, and tearing fracture energy of 0.4-18.5 kJ/m(2). We found that dual-cross-linking of the kappa-carrageenan network, i.e., the coiled-coil junctions and the metal-coordination bonds, was indispensable for the combined mechanical properties and stability of these gels. Essential reasons were rationally discussed based on the results of control experiments. The strategy we described should be applicable to other biopolymer-based hydrogels toward improved mechanical properties and stability, which may promote the applications of tough hydrogels in diverse areas.