Electrode catalyst layer, membrane electrode assembly, and polymer electrolyte fuel cell

An object is to provide an electrode catalyst layer, a membrane electrode assembly, and a polymer electrolyte fuel cell that can suppress decrease in durability of the membrane electrode assembly and decrease in power generation performance of the polymer electrolyte fuel cell by suppressing crack generation in the electrode catalyst layer. An electrode catalyst layer according to one aspect of the present invention is an electrode catalyst layer including at least: a catalytic substance; aggregates of polymer electrolytes; and polymer electrolyte fibers. In the electrode catalyst layer, an amount of phosphorus and an amount of platinum defined via elemental analysis by energy dispersive X-ray spectroscopy (EDX) satisfy a following equation (1) 0 P/Pt 3.0 Equation (1).