Inspired by photosynthesis, solar water electrolysis uses sunlight to produce hydrogen, a clean, sustainable and storable fuel and a promising solution to the energy storage conundrum. Semiconductor silicon has been the basis for several generations of photovoltaic cells but its use in artificial photosynthetic devices is still in its infancy. Recent progress in this area encompasses nanostructured forms of silicon including anodised porous silicon, silicon nanoparticles and silicon nanowires. At the same time, efforts are underway to explore biosilica-derived silicon from marine diatoms for photocatalysis. It is anticipated that by using these nanostructured materials, the solar conversion efficiency as well as cost effectiveness of these devices will be greatly improved. Porous silicon, porous silicon nanoparticles and silicon nanowires have been fabricated based on hydrofluoric acid etching techniques, whereas diatom-derived silicon is a naturally nanostructured form of silica, which can be converted into silicon using a magnesiothermic processes. At present, the performance of synthetic silicon nanostructures is approaching that achieved by conventional solar water splitting technology, producing photocurrent densities of up to 36mA/cm2. The device performance reported for diatom-derived silicon to date has been far lower (14µA/cm2) but new electrode fabrication techniques offer significant scope for improvement. This review explores the fabrication, use and potential impact of diverse silicon nanoarchitectures for solar water electrolysis.