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Hierarchical structures of 20 nm grains embedded with twins are realized in electrodeposited Au−Cu alloys. The electrodeposition method allows refinement of the average grain size to 20 nm order, and the alloying stabilizes the nanoscale grain structure. Au−Cu alloys are face-centered cubic (FCC) metals with low stacking fault energy that favors formation of growth twins. Due to the hierarchical structure, the Hall−Petch relationship is still observed when the crystalline size (average twin space) is refined to sub 10 nm region. The yield strength reaches 1.50 GPa in an electrodeposited Au−Cu alloy composed of 16.6 ± 1.1 nm grains and the average twin spacing at 4.7 nm.