This electrode catalyst layer for fuel cells is provided with: an electrode catalyst that comprises a conductive carrier and platinum-containing metal particles supported on the surface of the conductive carrier; and an ionomer that covers the electrode catalyst. This electrode catalyst layer for fuel cells is characterized in that the average thickness of the ionomer is 2.4 nm or less. This electrode catalyst layer for fuel cells is capable of having a good balance between proton transport properties and transport properties for a gas such as an oxidant gas or a fuel gas even in cases where the amount of supported platinum is decreased. In addition, an electrode for fuel cells, a membrane electrode assembly for fuel cells, and a fuel cell, each having good current-voltage characteristics, can be obtained using the above-described electrode catalyst layer for fuel cells.