Biomineralization is a biogenic process that produces elaborate inorganic and organic hybrid materials in nature. Inspired by the natural process, this study explored a new mineralization approach to create nanostructured CaCO films composed of amorphous CaCO hemispheres by using catechol-rich polydopamine (PDA) as a biomimetic mediator. The thus synthesized biomimetic CaCO was successfully transformed to nanostructured films of metal-oxide minerals, such as FeOOH, CoCO , NiCO , and MnOOH, through a simple procedure. The CaCO -templated metal-oxide minerals functioned as efficient electrocatalysts; a CaCO -templated cobalt phosphate (nanoCoPi) film exhibited high stability as a water-oxidation electrocatalyst with a current density of 1.5 mA cm . The nanostructure of nanoCoPi, consisting of individual nanoparticles (≈70 nm) and numerous internal pores (BET surface area: 3.17 m  g ), facilitated an additional charge-transfer pathway from the electrode to individual active sites of the catalyst. This work demonstrates a plausible strategy for facile and green synthesis of nanostructured electrocatalysts through biomimetic CaCO mineralization.