A multiblock copolymer includes a polysulfone repeating unit of formula (1) below; a sulfonated polysulfone repeating unit of formula (2) below; and a polydialkylsiloxane repeating unit of formula (3) below, a method of preparing the multiblock copolymer, a polymer electrolyte membrane prepared from the multiblock copolymer, a method of preparing the polymer electrolyte membrane, and a fuel cell including the polymer electrolyte membrane: where l, m, and n are integers of 1-200; each of R1 through R8 is independently hydrogen, fluorine, or a C1-C10 alkyl group that can be substituted by at least one fluorine atom; and X represents a tetraalkylamine cation that can be substituted by hydrogen ions or other cations by ion exchange. The multiblock copolymer has a high ionic conductivity, high hydrophobicity, and good mechanical properties, can minimize crossover of methanol, and can be manufactured at a low cost. In addition, the structure of the multiblock copolymer can be varied to increase selectivity to a solvent used in a polymer electrolyte membrane.