<p>Pure copper and brass were successfully joined using liquid CO₂-assisted cold source-assisted friction stir welding (CSA-FSW). Optical microscopy, electron backscatter diffraction techniques, and nanoindentation testing were used to characterize the microstructure and micromechanical properties within the stir zones on both the copper and brass sides. The results show that the dissimilar copper-brass joint with well-formed surfaces and free of internal defects was produced. The stir zone exhibited an ultra-fine grain structure on both the brass and copper sides. The number fractions of high-angle grain boundaries and twin boundaries within the stir zone on the brass side were increased by 34% and 13% compared to those for the copper side. The stir zone on the brass side exhibited a higher Taylor factor, lower dislocation density, finer grain size, and lower texture intensity than the copper side. The microhardness, yield strength, and strain-hardening index on the brass side were 1.36 GPa, 300&#xa0;MPa, and 0.36, respectively, which were increased by about 10%, 30%, and 0.14 compared to the copper side. The grain boundary strengthening and the dislocation strengthening were the main strengthening mechanisms that affected the mechanical properties of the dissimilar copper and brass joint.</p>