Lluest-wen Reservoir (Welsh: Cronfa-ddwr Lluest-wen) is a reservoir at the top of the Rhondda Fach Valley (Welsh: Cwm Rhondda Fach), in the borough of Rhondda Cynon Taf, South Wales. Parts of it fall within the communities of Rhigos, Aberdare and Treherbert. A recreational walking route, the Coed Morgannwg Way, and route 47 of the National Cycle Network run northeast and southwest of it respectively. The reservoir was built in 1898 and covers an area of 20 acres.