Provided is a functional polymer membrane including a porous support; and a resin layer and an auxiliary layer which are supported by the porous support, in which both of the resin layer and the auxiliary layer contain an ion exchange polymer having one of an anion exchange group and a cation exchange group, a charge of the ion exchange group of the ion exchange polymer included in the resin layer is opposite to a charge of the ion exchange group of the ion exchange polymer included in the auxiliary layer, and both of the ion exchange polymers respectively included in the resin layer and the auxiliary layer are polymers having a unit obtained from an acryloyl group which may have an alkyl group at the α-position; a stack or a device provided with a functional polymer membrane; and a method of producing, a functional polymer membrane.