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SrTiO₃ can be utilized as a photocatalyst for water splitting owing to its suitable band edge positions, thermal stability and low cost. However, it could not harness the visible light due to its wide band gap (3.25 eV). Doping is one of the prominent solutions to tailor the band gap and inducing the visible light response and thus, enhancing the photocatalytic activity. We address the role of monodoping of a metal (Mn) and a non-metal (N), as well as their codoping in SrTiO₃ for photocatalytic water splitting using state of the art density functional theory and ab initio atomistic thermodynamics. N_O reduces the band gap, but introduces the deep trap states that degrade the photocatalytic efficiency. Mn_Sr could not reduce the band gap of SrTiO₃. Mn_Ti could reduce the band gap, but it lowers down the conduction band edge and hence, lowers the reduction power. Our results reveal that the Mn_SrN_O (codoping of Mn at Sr and N at O in SrTiO₃) is the promising candidate as a photocatalyst for water splitting as it induces the visible light response as well as passivates the deep level states and its band edge positions straddle the redox potential of water.