The present invention provides: a polymer electrolyte composition which can achieve excellent proton conductivity under slightly humidified conditions, excellent mechanical strength and excellent physical durability, has excellent practicality, and can be produced using a nitrogen-containing additive, wherein the nitrogen-containing additive can prevent the elution of the additive under a strongly acidic atmosphere during the operation of a fuel cell, has excellent chemical stability so as to tolerate a strongly acidic atmosphere, can be dissolved in various general-purpose organic solvents, has superior processability, can be mixed with an ionic-group-containing polymer, can prevent the occurrence of phase separation during the formation of a film, and can prevent the formation of an island-in-sea-like phase separation structure or the occurrence of bleeding out during the formation of a film; and a polymer electrolyte membrane, a membrane electrode assembly and a polymer electrolyte fuel cell, each of which is produced using the polymer electrolyte composition. The polymer electrolyte composition according to the present invention comprises at least an ionic-group-containing polymer (A) and a nitrogen-containing additive (B), said polymer electrolyte composition being characterized in that the nitrogen-containing additive (B) is represented by a specific structural formula.