Mann Lake is the largest of the shallow playa lakes in the northern part of the Alvord Valley in Harney County in the U.S. state of Oregon. The lake is a remnant of a larger Pleistocene lake that covered much of the valley, east of Steens Mountain. Fields–Denio Road runs along the east side of the lake. Mann Lake, stocked with Lahontan cutthroat trout, has been called "one of the premiere trout fisheries in southeast Oregon". Trout in the lake range in length from 12 to 20 inches (30 to 51 cm). Fishing is restricted to artificial flies and lures.