Combining both chemical and physical crosslinks in a double-network hydrogel (DN gel) has emerged as a promising design strategy to obtain highly mechanically strong hydrogels. Unlike chemically crosslinked DN gels, little is known about the fracture process and toughening mechanisms of hybrid chemically-physically linked DN gels. In this work, we engineered tough hybrid DN gels of agar/polyacrylamide (Agar/PAAm) by combining two types of cross-linked polymer networks: a physically-linked, first agar network and a chemical-linked, second PAAm network. The resulting Agar/PAAm exhibited high stiffness of 313 kPa and high toughness of 1089 J/m². We then specifically examined the effect of the first agar network on the mechanical properties of hybrid Agar/PAAm gels. We found that by controlling agar concentrations above a critical value, the physically-linked agar network can simultaneously enhance both stiffness and toughness of Agar/PAAm DN gels, as evidenced by a linear relationship of elastic modulus and tearing energies of the gels as the increase of agar concentration. This toughening behavior is different from that of chemically-linked DN gels. Complement to chemically-linked DN gels, this work provides a different view for the design of new stiff and tough hydrogels using hybrid physical and chemical networks.