Hydrogel-forming composition and high strength hydrogel formed from same

[Problem] To provide: a self-supporting hydrogel which is highly strengthened and is able to be produced merely by mixing at room temperature; and a method for producing this hydrogel. [Solution] A hydrogel-forming composition which is capable of forming a self-supporting hydrogel, and which is characterized by containing (A) a water-soluble organic polymer having an organic acid structure, an organic acid salt structure or an organic acid anion structure, (B) a silicate salt, (C) a compound having a diphosphonic acid structure represented by formula (1) (wherein each of R 1 and R 2 independently represents a specific substituent; each of X 1 and X 2 independently represents a single bond or a linking group; and n represents an integer of 1-5) or a salt thereof, and (D) a compound which has or generates a divalent or higher-valent positive charge; a hydrogel which is formed from the composition; and a method for producing this hydrogel.