It has been hypothesized that a thermodynamically feasible Ag2Se monolayer could be a potential candidate for photocatalytic water splitting. However, the present electronic structure knowledge is insufficient for forecasting and confirming the ultimate criterion of photocatalysis. Its wide band gap of around 2.70 eV is also non-ideal for photovoltaic conversion. These challenges are addressed herein using first-principles density functional theory (DFT) calculations to systematically probe the photocatalytic potential, light absorption coefficient, carrier mobility and carrier utilization efficiency of Ag2Se. To ascertain the spontaneity of the solar-to-hydrogen conversion, significant efforts have been made to calculate the energy barriers for the surface hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Fascinatingly, upon -6% compressive biaxial straining, the Ag2Se monolayer ingeniously combines all the desired characteristics for photocatalytic water-splitting activity, including a broader sunlight absorption region (and SIM;10(5) cm(-1)), enhanced carrier mobility (and SIM;10(5) cm(2) V-1 s(-1)) and spontaneous catalytic pathways with relatively low triggering external potential, while retaining a direct band gap, good thermal stability and perfect band edge position. The corrected STH efficiency of and SIM;10% suggests commercial hydrogen production. This work provides valuable insights into the understanding of the catalytic mechanisms in the strain-modulated Ag2Se monolayer.