Lake Lanier (officially Lake Sidney Lanier) is a reservoir in the northern portion of the U.S. state of Georgia. It was created by the completion of Buford Dam on the Chattahoochee River in 1956, and is also fed by the waters of the Chestatee River. The lake encompasses 38,000 acres (150 km2) or 59 square miles of water, and 692 miles (1,114 km) of shoreline at normal level, a “full summer pool” of 1,071 feet (326 m) above mean sea level. It was named for poet Sidney Lanier, and was built and is operated by the U.S. Army Corps of Engineers. It is patrolled by the Georgia Department of Natural Resources (GDNR).

The lake is in Hall, Forsyth, Dawson, Gwinnett, and Lumpkin counties, split about 60%, 30%, 5%, 4%, and 1% respectively, filling the valley into numerous small arms and fingers. The former thalweg of the Chestatee and the Chattahoochee south of it form the county line between Hall and a tiny corner of Gwinnett to the east, and Dawson and Forsyth to the west.

One of the main purposes of the lake is flood control downstream of the lake, mainly protecting metro Atlanta. There have only been three major flooding events on the downstream section since the construction of Buford Dam. The most recent flooding event was in 2013.

The lake is popular with boaters, houseboats, jetskiers and others, particularly around the summer holidays. Over 7.5 million people per year visit the lake, including its marinas and the Lake Lanier Islands waterpark. The rowing and sprint canoeing events during the 1996 Summer Olympics were held at the lake. It also hosted the canoe sprint World Championships in 2003 at the Lanier Canoe and Kayak Club.

Established in 1957, Holiday Marina has become synonymous with Lake Lanier. Holiday Marina in Buford, Georgia, and Aqualand Marina near the town of Flowery Branch, are two of the largest freshwater marinas in the world.