Robust bonding of a hydrogel in aqueous environment, either to another hydrogel or to a solid, is one of the major unsolved issues for the practical applications of hydrogels in various fields. Here we report robust bonding between a pair of hydrogel sheets, containing over 90 wt% of water, by 10 applying the double-network (DN) structure. In the optimal condition, the peeling energy of the united gel sheets reaches 1200 J/m2, which is comparable to the bulk fracture energy of a normal type of tough DN gels. This hydrogel bonding technique is also applied to form tough bonding between hydrogel and plastic plates. Furthermore, based on this technique, we have developed a facile method to synthesize robust double network hydrogels with any desirable free-shape from 15 micro-gel precursors. These novel techniques will substantially merit the applications of the tough hydrogels in various fields, such as an artifical meniscus.