Disclosed are: a polyethersulfone polymer electrolyte which has excellent durability (particularly swelling resistance) while keeping its high ion conductivity, and which can be produced at a low cost; an electrolyte membrane; and a method for producing the polyethersulfone polymer electrolyte. The polyethersulfone polymer electrolyte has both of a constituent unit represented by the formula (I) and a constituent unit represented by the formula (II). wherein n and n' independently represent an integer of 1 to 6; and p, q, r, s, p', q', r' and s' independently represent an integer of 0 to 4, provided that all of these symbols cannot be 0 in a single molecule.
