Mackay Reservoir, reservoir on the Big Lost River in Custer County, Idaho, United States

Mackay Reservoir is a reservoir on the Big Lost River in Custer County, Idaho, held back by Mackay Dam, a 67-foot-high earthfill structure completed in 1918. It covers 1,392 acres in the Lost River Valley about 4 miles northwest of Mackay, between the Pioneer Mountains and the Lost River Range at the base of Mt. McCaleb. U.S. Route 93 runs along its eastern side.

The dam was constructed in 1918 to support irrigation in the valley and stands about 67 feet tall. Mackay purchased the facility in 1936, after which it became central to local identity and has withstood significant challenges including a major earthquake in 1983 that tested its structural integrity.

Locals call the reservoir "The Res." Since 1936, when the town of Mackay bought the dam, the community has held an annual fall barbecue to mark the purchase. Every Fourth of July, Mackay and Arco stage a fireworks show over the water.

The reservoir lies about 4 miles (6.4 km) northwest of Mackay, with U.S. Route 93 running along its eastern shore. Boating, fishing, camping and hunting are possible on the surrounding land, and the Idaho Department of Fish and Game stocks the water annually. The dam face stands 67 feet (20 m) high at the base of Mt. McCaleb, between the Pioneer Mountains and the Lost River Range.

In June 1933, unknown persons dynamited the reservoir's control tower and diversions, apparently because drought-stricken farmers blamed the dam for low water. The main dam structure was undamaged, but the attack pushed the Utah Construction Company to lower its selling price, and Mackay bought the reservoir in 1936.