Oxygen evolution from water is one of the key reactions for solar fuel production. Here, two nanostructured K-containing δ-MnO2 are synthesized: K-δ-MnO2 nanosheets and K-δ-MnO2 nanoparticles, both of which exhibit high catalytic activity in visible-light-driven water oxidation. The role of alkaline cations in oxygen evolution is first explored by replacing the K+ ions in the δ-MnO2 structure with H+ ions through proton ion exchange. H-δ-MnO2 catalysts with a similar morphology and crystal structure exhibit activities per surface site approximately one order of magnitude lower than that of K-δ-MnO2, although both nanostructured H-δ-MnO2 catalysts have much larger Brunauer–Emmett–Teller (BET) surface areas. Such a low turnover frequency (TOF) per surface Mn atom might be due to the fact that the Ru2+(bpy)3 sensitizer is too large to access the additional surface area created during proton exchange. Also, a prepared Na-containing δ-MnO2 material with an identical crystal structure exhibits a TOF similar to that of the K-containing δ-MnO2, suggesting that the alkaline cations are not directly involved in catalytic water oxidation, but instead stabilize the layered structure of the δ-MnO2.