Abstract Tantalum–oxide based compounds of the K2[Lnn−1TanO3n+1] cation-deficient Ruddlesden–Popper series are promising compounds as water splitting photocatalysts for hydrogen production. In this work the role of the lanthanide (Ln=La, Pr and Nd) in the K2LnTa3O10 tantalate for the photocatalytic mechanism is studied. The question arises as to whether it can be considered only as a “spectator” or if it contributes to this mechanism. The band gaps for the aforementioned tantalates are studied and here we evaluate how the localized Ln:4f states are positioned in the gap. This system is analyzed with electronic structure calculations using a linearized augmented plane wave method based on Density Functional Theory. It was found that two of the lanthanide atoms, Pr and Nd, contribute with 4f state in the gap, and these states act as ‘stepping stones’ for the electrons to jump to the conduction band. This can be useful in designing and tuning future materials for hydrogen production in the semiconductor-mediated photocatalytic process of water splitting.