We report single-atom doping of gold nanoclusters (NCs), and its drastic effects on the optical, electronic, and catalytic properties, using the 25-atom system as a model. In our synthetic approach, a mixture of Pt1Au24(SC2H4Ph)(18) and Au-25(SC2H4Ph)(18) was produced via a size-focusing process, and then Pt1Au24(SC2H4Ph)(18) NCs were obtained by selective decomposition of Au-25(SC2H4Ph)(18) in the mixture with concentrated H2O2 followed by purification via size-exclusion chromatography. Experimental and theoretical analyses confirmed that Pt1Au24(SC2H4Ph)(18) possesses a Pt-centered icosahedral core capped by six Au-2(SC2H4Ph)(3) staples. The Pt1Au24(SC2H4Ph)(18) cluster exhibits greatly enhanced stability and catalytic activity relative to Au-25(SC2H4Ph)(18) but a smaller energy gap (E-g approximate to 0.8 eV vs 1.3 eV for the homogold cluster).