Selenium nanoparticle-decorated silicon nanowire (Se NPs@Si NWs) electrodes are applied as a photoanode in a liquid-junction photoelectrochemical (PEC) solar cell for the first time. Upon illumination, the Se NPs anchored along the axial length of Si NWs allow fast hole extraction at the radial Se/Si junctions because of the p-type conduction nature of Se NPs, thus enhancing electron-hole separation and simultaneously increasing the population of photoexcited electrons in Si NWs through light scattering that amplifies the effective light absorption of Si NWs. These attributes of Se NPs result in a power conversion efficiency (PCE) of 7.03% for the Se NPs@Si NW-based liquid-junction solar cell encompassing a Br-/Br2 electrolyte and a carbon fabric counter electrode. This PCE is greater by 43% than that of the analogous Si NW-based cell. Se NPs are photoconducting because of facile hole propagation that occurs particularly along the c-axis of trigonal Se NPs with a hexagonal crystal structure and size effects improve the optical path length, factors that lead to a significantly improved performance. Compared to Pt or Au NPs that have been explored previously in combination with Si NWs, where their roles are distinctively different, Se NPs here are not only more cost effective and easy to be synthesized on a large scale but also enable an improvement in PCE of Si NWs by relying on unique mechanisms. Optical, structural, PEC, and impedance studies furnish a deep understanding of the phenomena involved in yielding a superior performing liquid-junction PEC solar cell based on the Se NPs@Si NW photoanode. © 2019 American Chemical Society.