Nitrogen doping into a series of layered tantalates (ALaTa2O7, where A = Li, Na, K, Rb, or Cs) was attempted in order to produce materials capable of catalyzing non-sacrificial and endergonic water reduction under visible light. Heating of KLaTa2O7 and RbLaTa2O7 in an NH3 stream at 1073 K led to successful nitrogen doping, accompanied by a significant shift in the absorption edge of the material toward the visible-light region, while similar treatment of the other tantalates resulted in the collapse of the layered structure or partial anion substitution at the surface. Although the NH3 heating of a conventional RbLaTa2O7 precursor prepared with nearly stoichiometric Rb (Rb/La = 1.2) resulted in the formation of impurities such as Ta3N5 and amorphous tantalum nitrides, the use of a Rb-rich precursor prepared with excess Rb (Rb/La = 2.4) effectively suppressed this impurity formation. The Rb+ cations in the prepared pure nitrogen-doped sample were exchanged with H+ to facilitate the intercalation of water, and a cationic Pt precursor was then selectively introduced into the interlayers and photocatalytically reduced to Pt metal particles. The internally platinized H+/RbLaTa2O7-xNy showed stable H2 evolution in the presence of I- as an electron donor under visible light, accompanied by the generation of I3 -. Although the externally platinized H+/RbLaTa2O7-xNy sample and other bulk-type photocatalysts such as Ta3N5 generated H2 in the presence of a sacrificial electron donor, H2 evolution was negligible in the presence of I-. The stable H2 evolution over the internally platinized H+/RbLaTa2O7-xNy sample is due to the suppressed backward reduction of I3 - to I- at selective reduction sites in the interlayer spaces, which are accessible only to cationic species and water.