Ruddlesden–Popper perovskites, HLaTiO4 and HLaTiO4−xNx were prepared by proton exchange of layered KLaTiO4 synthesized by a solid-state reaction and KLaTiO4−xNx obtained by nitridation, respectively. The influence of nitridation time on crystal structure, morphology, and absorption wavelength and optical band-gap energy of KLaTiO4 crystals was studied. According to the XRD and SEM results, the crystal structure and plate-like morphology of the parent oxide were roughly retained even after nitridation at 800 °C for 10 h. The absorption edge wavelength of the KLaTiO4 crystals was found to be at about 350 nm (Eg = 3.54 eV), while the absorption edge wavelength of the KLaTiO4−xNx crystals (after 10 h nitridation) was about 586 nm (Eg = 2.12 eV). To investigate the effect of partial N3−-to-O2− substitution and H+-to-K+ exchange on photocatalytic water reduction activity of Ruddlesden–Popper layered perovskite KLaTiO4, the photocatalytic activity for water reduction half-reaction over Pt-photodeposited KLaTiO4, KLaTiO4−xNx, HLaTiO4, and HLaTiO4−xNx was evaluated under simulated solar light. Among all the samples, Pt-photodeposited KLaTiO4 exhibited the highest photocatalytic activity for H2 evolution. In contrast, Pt-photodeposited KLaTiO4−xNx showed a low photostability and photocatalytic activity for H2 evolution due to the negative impact of the defective layer and reduced titanium species. In addition, perovskite oxynitride [LaTiO4−xNx]− nanosheets were successfully fabricated by a mechanical exfoliation (sonication) of the KLaTiO4−xNx crystals. The colloidal suspension of the oxynitride nanosheets showed a Tyndall effect, implying their good dispersion and stability in water.