Llyn Fawr ('large lake' in Welsh) is a reservoir that occupies one of a series glacial cirques that form the northern escarpment of the South Wales Coalfield uplands, overlooking the heads of the Vale of Neath and the Cynon Valley, South Wales.

It lies on the northern flanks of Craig-y-Llyn, a mountain that is designated as a Site of Special Scientific Interest.

While in government terms, it falls in the unitary authority of Rhondda Cynon Taf, in geographic terms it is actually in the Vale of Neath and its excess waters (all its water before conversion) flow into Nant Gwrelych, which flows into the River Neath at Pont Walby near Glynneath.

Owing to its rapid and massive industrialisation during its coalfield boom (population increasing from 951 in 1851 to 113,735 in 1901), the Rhondda Valley, which lies on the south side of the escarpment, surpassed the supply of water it could source from its own river, so in 1909 the Water Works division of Rhondda Urban District Council contracted builders to convert Llyn Fawr into a reservoir.

[6] Although it is subsiding due to collapsing coal working below, with water now pooling in the middle,[7] investigation is being made into ways to either stabilise it or replace it.