Llyn Cwm Llwch (Welsh: [ɬɪn kʊm ɬuːχ]) is a small lake or pool in the Brecon Beacons National Park in Powys, Wales.

It is of glacial origin, occupying a rock hollow beneath the peaks of Pen y Fan and Corn Du in the central Brecon Beacons.

It is drained by the Nant Cwm Llwch which empties into the Afon Tarell, which itself enters the River Usk at Brecon.

[1] Tommy Jones' obelisk, a memorial in granite to a young boy who died near this spot in 1900, overlooks the lake.

A popular path up to Pen y Fan runs southwards up Cwm Llwch, passing by the outlet of the lake.