Disclosed are: a polymer electrolyte membrane which can guarantee the production of a membrane-electrode assembly having excellent mechanical properties without a decrease in performance, such as in ionic conductivity, and thus having a high enough durability to achieve at least 30,000 wet/dry cycles as measured according to the NEDO protocol; a membrane-electrode assembly including the polymer electrolyte membrane; and a method for measuring the durability of the membrane-electrode assembly. The polymer electrolyte membrane according to the present invention comprises a composite layer including: a porous support having multiple pores; and ionomers filling the pores, and has an MD internal tearing strength of 150 N/mm or greater, a TD internal tearing strength of 150 N/mm or greater, a stab initial strain of 8% or less, and a stab final strain of 10% or less.