An adhesive hydrogel contains: a polymer matrix of a copolymer of a monofunctional monomer and a crosslinking monomer (a); another polymer component; water; and a polyhydric alcohol, wherein: either a structural unit derived from the monofunctional monomer contained in the polymer matrix or the other polymer component or both contain a structural unit derived from at least one compound (b) selected from the group consisting of ethylenically unsaturated carboxylic acids, ethylenically unsaturated carboxylic acid salts, and ethylenically unsaturated carboxylic acid derivatives having a hydroxyl group, an acid group, a salt thereof, an amino group, and/or an ammonium group; and the other polymer component contains a polymer compound (c) with a side chain having an oxyalkylene group or a polyoxyalkylene group.