Catalysts for the photogeneration of hydrogen from water are key for realizing solar energy conversion. Despite tremendous efforts, developing hydrogen evolution catalysts with high activity and long-term stability remains a daunting challenge. Herein, the design and fabrication of mesoporous Pt-decorated CdS nanocrystal assemblies (NCAs) are reported, and their excellent performance for the photocatalytic hydrogen production is demonstrated. These materials comprise varying particle size of Pt (ranging from 1.8 to 3.3 nm) and exhibit 3D nanoscale pore structure within the assembled network. Photocatalytic measurements coupled with UV-vis/NIR optical absorption, photoluminescence, and electrochemical impedance spectroscopy studies suggest that the performance enhancement of these catalytic systems arises from the efficient hole transport at the CdS/electrolyte interface and interparticle Pt/CdS electron-transfer process as a result of the deposition of Pt. It is found that the Pt-CdS NCAs catalyst at 5 wt% Pt loading content exerts a 1.2 mmol h(-1) H-2-evolution rate under visible-light irradiation (lambda >= 420 nm) with an apparent quantum yield of over 70% at wavelength lambda = 420 nm in alkaline solution (5 m NaOH), using ethanol (10% v/v) as sacrificial agent. This activity far exceeds those of the single CdS and binary noble metal/CdS systems, demonstrating the potential for practical photocatalytic hydrogen production.