Abstract: Commercial hydrogen production involves the development of efficient hydrogen evolution reaction catalysts. Herein, we adopted a friction stir processing (FSP) technique to mix immiscible metals homogenously and obtain a self-supporting copper–silver (CuAg) catalyst. The gust of Ag atoms with larger atomic sizes caused a tensile strain in the Cu matrix. Meanwhile, the chemical-potential difference induced electron transfer from Cu to Ag, and the two factors jointly led to the upshift of Cu d-band and improved the catalytic activity. Consequently, the CuAg electrode exhibited a high turnover frequency (12 times that of pure Cu), a low overpotential at high current density (superior to platinum foil), and high durability (1.57% decay over 180 h). Our work demonstrates that FSP is a powerful method for preparing self-supporting catalysts of immiscible alloys with high catalytic performance.