Nanocrystalline spinel LiMn₂O₄ has been prepared and treatment of LiMn₂O₄ with dilute nitric acid solution resulted in the delithiation of the framework, while maintaining the spinel structure, λ-MnO₂. LiMn₂O₄ is not a catalyst for water oxidation. Upon removal of the lithium, the cubical Mn₄O₄ cores become active sites for oxidizing water to molecular oxygen, which was investigated with the photochemical [Ru²⁺(2,2′-bpy)₃]/persulfate system at pH 5.8. The nanosize λ-MnO₂ obtained from the nanocrystalline LiMn₂O₄, which was synthesized by the citrate route, shows a significantly higher water oxidation catalytic activity (Turnover Frequency: 3 × 10⁻⁵ mol O₂/s/mol Mn) than that obtained via solid state reaction with micrometer and irregular particle sizes (Turnover Frequency: 5 × 10⁻⁶ mol O₂/s/mol Mn).