Provided are a hydroxide ion-conductive membrane including a porous substrate and a hydroxide ion-conductive polymer disposed at least in pores of the porous substrate and having a thickness of the hydroxide ion-conductive membrane of 5 μm or more and less than 50 μm, in which the polymer has 50% by mole or more of a constituent component (I) derived from a polyfunctional polymerizable monomer having a total of two or more atoms of at least one of an oxygen atom, a sulfur atom, or a nitrogen atom in a structural moiety other than a polymerizable group in constituent components of the polymer, and a method for producing the hydroxide ion-conductive membrane, and a membrane electrode assembly, and a method for producing hydrogen and a hydrogen production system, each using the membrane electrode assembly.