Provided are a tri-block copolymer and an electrolyte membrane prepared therefrom. The tri-block copolymer has a structure of polar moiety-containing copolymer block/non-polar moiety-containing copolymer block/polar moiety-containing copolymer block, or non-polar moiety-containing copolymer block/polar moiety-containing copolymer block/non-polar moiety-containing copolymer block, and is useful for an electrolyte membrane for fuel cells. The electrolyte membrane for fuel cells prepared from the tri-block copolymer exhibits superior dimensional stability and excellent fuel cell performance.