To provide an electrocatalyst for fuel cells, which is configured to ensure both the initial performance and durability of fuel cells. An electrocatalyst for fuel cells, wherein the electrocatalyst comprises a carbon support including a mesopore and a catalyst alloy supported on the carbon support, and the catalyst alloy is a catalyst alloy of platinum and a transition metal; wherein the mesopore includes at least one throat; wherein an average effective diameter of the at least one throat is 1.8 nm or more and less than 3.2 nm; and wherein a transition metal ratio of the catalyst alloy supported on a deeper-side region than the at least one throat, is lower than the transition metal ratio of the catalyst alloy supported on a nearer-side region than the at least one throat.