Controlling oxygen deficiencies is essential for the development of novel chemical and physical properties such as high-Tc superconductivity and low-dimensional magnetic phenomena. Among reduction methods, topochemical reactions using metal hydrides (e.g., CaH₂) are known as the most powerful method to obtain highly reduced oxides including Nd_0.8Sr_0.2NiO₂ superconductor, though there are some limitations such as competition with oxyhydrides. Here we demonstrate that electrochemical protonation combined with thermal dehydration can yield highly reduced oxides: SrCoO_2.5 thin films are converted to SrCoO₂ by dehydration of HSrCoO_2.5 at 350 °C. SrCoO₂ forms square (or four-legged) spin tubes composed of tetrahedra, in contrast to the conventional infinite-layer structure. Detailed analyses suggest the importance of the destabilization of the SrCoO_2.5 precursor by electrochemical protonation that can greatly alter reaction energy landscape and its gradual dehydration (H_1–xSrCoO_2.5–x/2) for the SrCoO₂ formation. Given the applicability of electrochemical protonation to a variety of transition metal oxides, this simple process widens possibilities to explore novel functional oxides.