The development of a solar-driven water splitting device that replaces costly noble metal electrodes, while achieving high performance and stable operation, is a major challenge. Transition metal phosphides (TMP) are being developed as low-cost hydrogen evolution reaction (HER) catalysts, but no reports have demonstrated their successful integration with a photoabsorber achieving stable performance in liquid junction electrolytes. Here, we report on a monolithic junction consisting of cubic-NiP₂ HER catalyst : TiN ultrathin-film : Si photoabsorber. Crystalline TiN creates an electron conducting, near-transparent film that effectively hinders atomic diffusion during fabrication and maintains stable interfaces. Crystalline cubic-NiP₂ on TiN/n⁺p-Si retains 97% of the bare Si’s photovoltage, a comparable J_sc to bare Si, and achieves a turnover frequency of 1.04 H₂ site^-1s⁻¹ at −100 mV applied electrical potential. When used in 0.5 M H₂SO₄ it requires only −150 mV overpotential beyond Pt/TiN/n⁺p-Si benchmark to reach an HER photocurrent density of −10 mA/cm². This photocathode maintains a stable H₂ photocurrent (±10%) for at least 125 hours, the duration of test. Transmission electron microscopy, voltammetry, ion scattering, AFM and XPS give information on the physical properties responsible for the observed activity and stable performance of these interfaces.