Provided is a hydrogel which comprises water as the main component at a high water content and has mechanical strength suitable for practical use and which exhibits high transparency and self-healing and shape-retaining properties. Also provided are both a material for the hydrogel and a novel polyionic dendrimer. A polyionic dendrimer which comprises a hydrophilic linear polymer as the core and polyester dendrons attached to both terminals of the linear polymer and in which cationic groups are bonded to the surfaces of the dendrons, said cationic groups being selected from the group consisting of guanidine group, thiourea group, and isothiourea group a material for a hydrogel, which comprises the polyionic dendrimer and clay and a hydrogel prepared using the material.
