Splitting water into hydrogen and oxygen is one of the most promising ways of storing energy from intermittent, renewable sources in the future. Toward this goal, development of inexpensive, stable, and non-toxic catalysts for water oxidation is crucial. We report that the electrodeposition of manganese oxide in the presence of sodium dodecyl sulfate (SDS) produces a material that is highly active for electrocatalytic water oxidation at pH near 7 and remains stable for over 24 hours of sustained electrolysis. Clark electrode measurements demonstrate more than 95% Faradaic efficiency for oxygen evolution after an initial charging period. We found that catalytic performance was optimized in films prepared by electrodeposition using a precursor solution containing moderate concentration of substrates, namely 25 mM Mn(2+) and 25 mM SDS. Microstructure and elemental analyses revealed that the deposited material, a mixed-phase manganese oxide, is structurally similar to materials used for electrochemical capacitors and batteries, drawing a parallel between highly studied cathode materials for rechargeable batteries and heterogeneous catalysts for water oxidation.