The Rhondda Fawr has its source on the eastern side of Craig y Llyn south of Llyn Fawr in a spring called Ffynnon y Gwalciau at an elevation of about 544m OD. The main head stream, Nant Carn Moesen (or Nant Carfoesen) runs down a plateau slope for about a mile and drops steeply, joining with the Nant Garreg-lwyd and the Nant Melyn to form the Rhondda. The combined waters then continue descending sharply through a mountain gorge to the foot of Pen Pych. The river runs through Blaenrhondda where it is joined by the Nant y Gwair, and runs down the Rhondda Valley, (). The river then passes through a string of mining towns and villages including Treherbert, Treorchy, Pentre, Ton Pentre, Ystrad Rhondda, Llwynypia, Tonypandy (joined by the Nant Clydach), Dinas and Porth where it joins its sister tributary.