The two-dimensional p-type semiconductor WSe2 is a promising candidate for photo-electrochemical (PEC) H-2 generation. Despite its high absorption coefficient, the PEC hydrogen evolution reaction (HER) efficiency is low and necessitates a catalyst to achieve higher photocurrents. Here, we investigate the monodisperse spherical alloy FePt and pure Pt nanocrystals (NCs) as catalysts for the PEC HER on p-WSe2 single-crystal photocathodes. After deposition of Pt or FePt, photocurrents of -0.27 and -4.0 mA cm(-2) at 0 V versus the reversible hydrogen electrode were achieved for the HER, which were 7.4 and 15 times higher compared to pristine WSe2 single crystals, respectively. Calculations at the density functional theory level show that the water adsorption and thus enhanced H2O dissociation are preferential on FePt in comparison to Pt. The edge sites of both Pt and FePt have a more negative adsorption energy than the (111) and (100) facets, indicating that the edges are the preferential sites for hydrogen production. The computational results show that the size of the Pt NC, within the range of Pt-55 (1.1 nm) and Pt-147 (1.6 nm), does not significantly influence the mechanism for the HER on the NCs.