Abstract: Mountain rivers serve as critical links between terrestrial eco-hydrological processes and aquatic ecological environments, playing a pivotal role in global hydrological cycles, material transport, and ecosystem maintenance. Given the heightened disaster risks and poorly defined characteristics of mountain rivers under climate change, this study systematically reviews research on mountain rivers to establish a clear scientific definition. Through comparative analysis of the geomorphological, hydrological, sedimentological, and ecological characteristics of mountain versus lowland rivers, this study identifies the defining features of mountain rivers as: surface relief ≥200 m/km², longitudinal channel gradient ≥2‰, gravel-bedrock dominated channel beds, and relatively narrow valleys. Based on an interdisciplinary perspective integrating geomorphology, hydrology, and ecology, a classification system and spatial identification framework for mountain rivers are proposed. Future research priorities should include refined identification of mountain rivers, delineation of management and control zones, identification of flood-obstructing elements, and quantitative assessment of flood conveyance risks. The research findings can provide technical support for the classified management and systematic governance of mountain rivers.