The present invention relates to a vinyl alcohol-based polymer having an olefin in side chain, comprising a vinyl alcohol unit and a structural unit represented by Formula (I), wherein X represents a divalent saturated hydrocarbon group with 1 to 6 carbon atoms, optionally having a branched structure; Y represents a hydrogen atom or a saturated hydrocarbon group with 1 to 6 carbon atoms, optionally having a branched structure; and Z represents a hydrogen atom or a methyl group. Thus, there is provided a vinyl alcohol-based polymer having an olefin in side chain exhibiting higher water solubility even after thermal treatment, higher storage stability, and higher reactivity to high energy beam, and a method for producing it.