Poulaphouca Reservoir, Artificial reservoir in County Wicklow, Ireland.

Poulaphouca Reservoir is a large artificial lake in County Wicklow created by damming the River Liffey. The water body stretches across a broad valley bordered by woodlands and hills, with recreational areas along its shores.

Work on the dam began in 1937, with the valley flooding starting in 1940 as part of Ireland's early hydroelectric development. This project represented a major step in the country's energy infrastructure at that time.

The name derives from Irish Poll a' Phúca, "the Púca's hole", after the Poulaphouca waterfall at the lake's south-western end where the water exits. The lake is commonly called the Blessington Lakes, although there is only one. Parts of the 1947 film Captain Boycott were shot in the lakeside villages of Lacken and Ballyknockan.

The Blessington Greenway, 6.5 km long, runs from Blessington village south along the lakeshore and forestry to Russborough House, crossing a medieval ringfort and using a stretch of footpath beside the N81 before turning back into forest at Burgage Moyle lane. A second phase aims at a 30 km loop around the reservoir for walking and cycling.

Between 1938 and 1940, 76 houses were demolished and the bridges at Humphreystown, Baltyboys and Burgage were blown up before the valley was flooded. The reservoir's irregular shape comes from flooding two river valleys, mainly that of the King's River, whose flow once exceeded the Liffey's at their confluence.