Platinum hydrogen evolution reaction (HER) electrocatalysts in the form of nanocubes (NCs) were synthesized at 50 °C by aqueous-based colloidal synthesis and were applied to electrochemical (EC) and photoelectrochemical (PEC) systems by a fast and simple drop-casting method. A remarkable Pt mass activity of 1.77 A mg−1 at −100 mV was achieved in EC systems (fluorine-doped tin oxide/Pt NC cathode) with neutral electrolyte while maintaining low overpotential and Tafel slope. In the Cu(In,Ga)(S,Se)2 (CIGS)-based PEC system, a carefully chosen amount of Pt NC loading to achieve a compromise between the catalytic activity (more Pt NCs) and better light transmittance (fewer Pt NCs) led to a maximum onset potential of 0.678 V against the reference hydrogen electrode. The photoelectrodes with Pt NCs also exhibited good long-term operational stability over 9.5 h with negligible degradation of the photocurrent. This study presents an effective strategy to greatly reduce the use of expensive Pt without compromising the catalytic performance because the drop-casting of Pt NC solutions to form electrocatalysts is expected to waste less raw material than vacuum deposition.