<p>Water electrolysis is gaining attention as a method for clean hydrogen production, making the development of efficient acidic oxygen evolution reaction (OER) catalysts crucial. MnO₂, with its diverse crystalline phases, has emerged as a promising candidate due to its stability and catalytic activity under acidic conditions. This review explores MnO₂-based catalysts, focusing on the role of crystal structure, morphology, valence states, and defect engineering in enhancing performance. The reaction mechanisms of OER, along with the transformation pathways of MnO₂ during the reaction, are examined to provide a deeper understanding of its catalytic behavior. Furthermore, based on these fundamental insights, recent advancements in MnO₂-based catalysts are reviewed, categorizing them into unsupported catalysts and MnO₂ as a support material, with discussions on noble- and non-noble-metal incorporation strategies for optimizing their practical application in acidic OER.</p>