Provided are (1) a vinyl alcohol graft copolymer grafting polymerizable unsaturated monomers as a graft chain onto a side-chain mercapto group-containing vinyl alcohol polymer backbone, the graft chain being attached to the polymer backbone at a mercapto group, and the backbone including: a vinyl alcohol unit and a unit represented by the following formula (I): wherein R1 is a hydrogen atom or a carboxyl group; R2 is a hydrogen atom, a methyl group, a carboxyl group, or a carboxymethyl group; and X is a divalent C1-22 group which contains a carbon atom and a hydrogen atom and may contain a nitrogen atom and/or an oxygen atom; and (2) an ion-exchange membrane having controlled swelling in water and excellent flex resistance, the membrane including a vinyl alcohol graft copolymer as described above.