Water-splitting semiconductor photocatalysts require hydrogen evolution reaction (HER) cocatalysts, the majority of which are robust metals and metal oxides. Metal complexes feature designability and reaction selectivity. This feature should be appreciated in the application as HER cocatalysts by suppressing unfavorable back reactions; however, low stability hampers their application. In this research, we demonstrate that a fully π-conjugated and conductive two-dimensional metal-organic framework (2D MOF), which possesses both virtues of metals/metal oxides and metal complexes, serves as an ideal HER cocatalyst candidate. NiBHT, a kind of 2D MOF, is deposited on the photocatalyst SrTiO3:Al with CoOxas an oxygen evolution cocatalyst to show long-term overall one-step water splitting with an apparent quantum efficiency (AQE) of 6.5% at 350 nm. In comparison to Pt as a representative HER cocatalyst, NiBHT turns out to be a suppressor of back reactions related to oxygen reduction reactions, which is proven both experimentally and theoretically.