A polymer membrane for a fuel cell includes a porous polymer substrate, and a crosslinked polymer layer having a proton-conductive functional group coated on at least one side of the porous polymer substrate. A method of preparing the polymer membrane includes: (1) providing a coating composition that includes a monomer or a prepolymer having the proton-conductive functional group and a crosslinking catalyst; (2) coating the coating composition on the at least one side of the porous polymer substrate; and (3) crosslinking the monomer or the prepolymer coated on the porous polymer substrate.