LaTaON ₂ is a promising visible-light-responsive photocatalyst for water splitting because of its broad visible light absorption and suitable band edge positions. However, the high defect concentration hinders the charge transfer and results in the poor photocatalytic performance of LaTaON ₂. Loading proper cocatalysts is one of the most efficient strategies for promoting charge separation/transfer and achieving high reaction activity. In this work, we have used density functional theory calculations to study the depositions of Pt, Ru and Ni single atom cocatalysts on LaTaON ₂(010) surface. The most stable adsorption configuration is the same site for all the elements, namely the top of the N atom on the La-terminated surface and the fourfold hollow site on the Ta-terminated surface. The adsorption of metal single atom on Ta-termination is stronger than that on La-termination due to the formation of more bonds. Upon the deposition, no localized impurity states appear in the middle of the forbidden gap since the n d states of metal adatoms are located within the valence band and conduction band of LaTaON ₂. The efficiency of the photocatalysts is probed by investigating their ability to adsorb H atom in a thermodynamically manner. Our results reveal that the energetically favorable sites of HER are the N atom on the La-termination and the O and N atoms on the Ta-termination, respectively. Compared with the clean surface, the surfaces with Pt, Ru and Ni single adatoms exhibit higher performance for HER because loading metal cocatalysts can further activate the surface nonmetal atoms and reduce the Gibbs free energy of hydrogen adsorption. The work gives an atom-level insight into the role of metal single atom cocatalysts in LaTaON ₂ photocatalyst for hydrogen production.