Reelfoot Lake is a shallow natural lake located in the northwest portion of Tennessee and extending into Fulton County, Kentucky , United States of America. Much of it is really more of a swamp, with bayou-like ditches (some natural, some man-made) connecting more open bodies of water called basins, the largest of which is called Blue Basin. Reelfoot Lake is noted for its bald cypress trees and its nesting pairs of bald eagles. It is the site of Reelfoot Lake State Park. Lake Isom, a similar, smaller lake to the immediate south, is a National Wildlife Refuge area.

Until 2003, Reelfoot was the world's only legal commercial fishery for crappie, which was served in restaurants located near the shore. It is the only large natural lake in Tennessee, and is the namesake of Lake County, Tennessee, in which it is primarily located. Poor agricultural practices have resulted in the siltation of the lake occurring at a far more rapid rate than it otherwise should, as it was common practice for cotton and soybeans to be planted literally up to the water's edge until governmental agencies purchased almost the entire shoreline and forbade the perpetuation of this practice. Siltation is still accelerated by the almost ubiquitous (in the area) custom of "burning out" the adjacent ditchlines every fall.

Popular history says that the lake was formed when the region subsided after the New Madrid earthquakes of 1811-1812, and that the Mississippi River flowed backward for 10-24 hours to fill it. The observations of the few persons in the region at time confirm that something serious occurred in the Reelfoot area in conjunction with the earthquakes, and that it undoubtedly resulted in major changes in the landforms of the area (which are very changeable at any rate, as the area is comprised primarily of glacial loess).

In the early 20th century the Reelfoot area was the site of widespread lawlessness and "Night Riding", which eventually resulted in the deployment of the state militia by governor of Tennessee Malcolm R. Patterson in 1908 to restore law and order. The troubles began when a group of landowners purchased almost the entire shoreline of the lake. They formed the West Tennessee Land Company to enforce what they saw to be their legal rights, including the ownership of the lake itself, and most importantly its fishing rights, as they owned the surrounding land. Most of the Night Riders were from families that had derived some or much of their living from fishing the lake for generations and their friends and supporters. Two attorneys engaged by the West Tennessee Land Company to enforce its claims fell into the hands of the Night Riders. According to a contemporary front-page account in the Nashville Banner, one was murdered by being both hanged and shot while the other escaped with his life to tell the story only by swimming a considerable distance across the lake at night, part of the while under fire from the Night Riders' guns. The publicity given this account led to a general consensus in the state government that action was required. Soon, the use of the lake was declared to be part of the state's public domain, guaranteeing the right of the public to use it whether they owned adjacent land or not. Later, a system of parks, wildlife refuges, recreation areas, and public boat ramps was developed around the lake through federal-state cooperation.

Since 1930 water levels in the lake have been somewhat regulated by the construction and operation of a spillway at the southern end of the lake where the Running Reelfoot Bayou flows out of it; this structure was controversial and an abortive attempt was made to blow it up by area residents in 1939 but it was not seriously damaged. The 1930 structure is now regarded as obsolete by both the U.S. Fish and Wildlife Service and the United States Army Corps of Engineers and plans are being made to replace it.

The lake is said to be named for a legendary Indian chief who had a deformed foot and was called "Reelfoot" by the whites, but this, too, is unproven. In his 1911 story "Fishhead," Irvin S. Cobb claimed the lake "[took] its name from a fancied resemblance in its outline to the splay, reeled foot of a cornfield negro."