Provided are: a practically excellent polymer electrolyte composition having excellent chemical stability of being resistant to strong oxidizing atmosphere during operation of fuel cell, and achieving excellent proton conductivity under low-humidification conditions, excellent mechanical strength and physical durability; a polymer electrolyte membrane, a membrane electrode assembly, and a polymer electrolyte fuel cell each using the same. The polymer electrolyte composition of the present invention comprises at least an ionic group-containing polymer (A) and a phosphorus-containing additive (B), the phosphorus-containing additive (B) being at least one of a phosphine compound and a phosphinite compound. The polymer electrolyte membrane, the membrane electrode assembly, and the polymer electrolyte fuel cell of the present invention are structured by the polymer electrolyte composition.