Abstract: River discharge is a core element of hydrological monitoring, and its timeliness and accuracy is of paramount importance for flood and drought disaster prevention, integrated water resources management, and the efficient operation of hydraulic and hydropower projects. Conventional stream gauging techniques, however, are often time-consuming, labor-intensive, and challenging to implement during high-flow events. Consequently, online discharge monitoring has emerged as a key research focus in the international hydrology community. A systematic review of the state-of-the-art in online discharge monitoring technologies was demonstrated. It analyzed the principles, research advances, applications, and limitations of mainstream techniques, categorizing them into five dimensions: space-based (satellite remote sensing), airborne (UAVs), ground-based (point/cableway radar, side-looking radar, image velocimetry), water-based (stationary ADCP, ultrasonic transit-time, acoustic tomography), and structure-based (hydraulic and gauging structures). Finally, the future trends, including enhancing sensing capabilities, multi-source data fusion, and the advancement of intelligent algorithms were outlined. This review aims to provide a valuable reference for improving river discharge monitoring system and to guide future research in this field.