The Rhymney River (Welsh: Afon Rhymni) is a river in the Rhymney Valley, South Wales, flowing through Cardiff into the Severn Estuary. The river formed the boundary between the historic counties of Glamorgan and Monmouthshire until in 1887, the parishes east of the river, Rumney and St Mellons, were transferred from the jurisdiction of Newport, to Cardiff in Glamorgan.
The river flows south from its source near Rhymney through New Tredegar, Bargoed, Ystrad Mynach, and Llanbradach to Caerphilly at the southern end of the Rhymney Valley. Then past Bedwas, Trethomas, Machen, Draethen, Llanrumney and Rumney and its estuary into the River Severn.