A surface-treated steel sheet of the present invention includes a base steel sheet and a Ni—Co—Fe alloy-plated layer on at least one surface of the base steel sheet, in which, in the alloy-plated layer, a Ni coating weight is 7.1 to 18.5 g/m2, a Co coating weight is 0.65 to 3.6 g/m2, and a total of the Ni coating weight and the Co coating weight is in a range of 9.0 to 20.0 g/m2. In a surface layer of the alloy-plated layer, a Co concentration is in a range of 20 to 60 atom %, and a Fe concentration is in a range of 5 to 30 atom %. In the alloy-plated layer, a region having a thickness of 2 μm or more, in which a total of a Ni concentration and the Co concentration is 10 atom % or more and the Fe concentration is 5 atom % or more, is present. The base steel sheet has a predetermined chemical composition, and a ferrite grain size number is 10 or more.