Ffynnon Lloer (Welsh for 'source of [the] Moon') is a lake in the Carneddau range of mountains in Snowdonia, North Wales.

It lies below the peaks of Pen yr Ole Wen and Carnedd Dafydd at a height of 2,225 feet (678 m) and covers an area of some 6 acres (2.43 ha).

The remains of two wartime aircraft, which crashed in 1942 and 1943, lie on the slopes round the lake.

[1] Afon Lloer, its outflow, flows into Llyn Ogwen.