AbstractIridium oxide is the state‐of‐the‐art catalyst for electrochemical water oxidation in an acidic medium. Despite being one of the rarest elements in the Earth's crust, there is a pressing need to maximize the utilization and longevity of active iridium centers. While conventional low‐temperature synthesis can yield nanostructures with high mass‐specific activity, they are often insufficiently stable during water oxidation. Structurally ordered iridium oxide is one of the most stable electrocatalysts utilized in polymer electrolyte membrane water electrolysis that benefits from the chemically ordered structure. However, its preparation requires thermal treatment at high temperatures, which improves its durability but can also result in reduced surface area and altered particle morphology. In this study, the challenge of controlling nanoparticle size during the preparation of structurally ordered iridium oxide is successfully addressed, which typically requires high‐temperature thermal treatment. By utilizing a silica nanoreactor as a hard template, a precise control is achieved over the nanoparticle size during high‐temperature thermal treatment. This approach maintains high durability while avoiding the common problem of reduced surface area and altered particle morphology. Specifically, this study is able to synthesize iridium oxide nanoparticles at temperatures up to 800 °C, while keeping their dimensions below 10 nm.