Substitution of oxide anions (O2-) in a metal oxide for nitrogen (N3-) results in reduction of the band gap, which is attractive in heterogeneous photocatalysis; however, only a handful of two-dimensional layered perovskite oxynitrides have been reported, and thus, the structural effects of layered oxynitrides on photocatalytic activity have not been sufficiently examined. This study reports the synthesis of a Ruddlesden-Popper phase three-layer oxynitride perovskite of K2Ca2Ta3O9N center dot 2H(2)O, and the photocatalytic activity is compared with an analogous two-layer perovskite, K2LaTa2O6N center dot 1.6H(2)O. Topochemical ammonolysis reaction of a Dion-Jacobson phase oxide KCa2Ta3O10 at 1173 K in the presence of K2CO3 resulted in a single-phase layered perovskite, K2Ca2Ta3O9N center dot 2H(2)O, which belongs to the tetragonal P4/mmm space group, as demonstrated by synchrotron X-ray diffraction, scanning transmission electron microscopy measurements, and elemental analysis. The synthesized K2Ca2Ta3O9N center dot 2H(2)O has an absorption edge at around 460 nm, with an estimated band gap of ca. 2.7 eV. K2Ca2Ta3O9N center dot 2H(2)O modified with a Pt cocatalyst generated H-2 from an aqueous solution containing a dissolved NaI as a reversible electron donor under visible light (lambda > 400 nm) with no noticeable change in the crystal structure and light absorption properties. However, the H2 evolution activity of K2Ca2Ta3O9N center dot 2H(2)O was an order of magnitude lower than that of K2LaTa2O6N center dot 1.6H(2)O. Femtosecond transient absorption spectroscopy revealed that the lifetime of photogenerated mobile electrons in K2Ca2Ta3O9N center dot 2H(2)O was shorter than that in K2LaTa2O6N center dot 1.6H(2)O, which could explain the low photocatalytic activity of K2Ca2Ta3O9N center dot 2H(2)O.