Pseudobinary ZnxCd1-xS semiconductor with zinc blende/wurtzite (ZB/WZ) twin-induced crystal structure has been regarded as one of the best pristine sulfide photocatalysts for H-2 evolution via water splitting. While the synthesis of this nano-twin photocatalyst is at the forefront of research, the incorporation of co-catalyst to improve photocatalytic activities through synergistic properties has rarely been reported. Loading well-dispersed ultrasmall platinum (Pt) with controlled sizes is the pivotal element in maximizing atom-utilization efficiencies by downsizing the noble metal clusters. In this contribution, we report the fabrication of highly dispersed sub-2 nm Pt decorated twinned Zn0.5Cd0.5S nanocrystals and their excellent photocatalytic H-2 evolution in both acidic and alkaline sacrificial reagents. The high visible-light-driven H-2 evolution rates of the most active sample (denoted as 8Pt-Zn0.5Cd0.5S) in 0.1 M Na2S/0.1 M Na2SO3 (pH = 12.94) and 0.15 M ascorbic acid (pH = 2.24) are measured to be 114.3 mu mol h(-1) (AQY:7.15%) and 164.9 mu mol h(-1) (AQY:8.56%), respectively, which render ca. 4.9- and 27.9-fold enhancement over pristine twinned Zn0.5Cd0.5S. The presence of homojunctions (within ZB and WZ segments) and heterojunctions (between Pt and twinned Zn0.5Cd0.5S) impart an efficient spatial charge separation and provide more active sites for highly efficient photocatalytic H-2 production.