A high efficiency, low band-gap photocatalyst is presented. The platform consists of gold nanoparticles (AuNPs) confined in the pores of a silicon substrate. Careful fabrication of the pores and choice of the particles allows for a maximum of two AuNPs within a single pore, preventing agglomeration. The nanoporous substrate is produced by first, creating a mask with block copolymer templating and metal infiltration on a silicon substrate, and second, reactive ion etching to remove the silicon, which has not been masked by the template. In this work, the pores are 60 nm in depth and 25 nm in diameter and the AuNPs are 18 nm in diameter. The AuNPs’ access to the analyte, provides more active sites for redox reaction, leading to enhanced efficiency. While proximity of nanoparticles enhances coupling efficiency, confinement prevents rapid recombination of photogenerated charge carriers, a major factor contributing to low efficiency of photocatalytic materials. Degradation of methyl orange (MO) is used to determine the photocatalytic efficacy of AuNSM compared to (i) bare silicon and (ii) AuNPs randomly dispersed on silicon. After 90 minutes exposure to UV light (λ = 353 nm) in the AuNSM, the MO absorption is <1%, indicating near complete degradation, while it is still 85% and 70% for systems (i) and (ii), respectively. Finite Element Method simulations of the confined structure suggest that the AuNPs act as a mediator/receptacle for photogenerated charges rather than a source of them at this wavelength and thus enhance the performance of the photocatalyst by creating more effective Schottky junctions—preventing recombination of electrons and holes—rather than by a localised surface plasmonic resonance effect.