Reelfoot Lake is a shallow natural lake located in the extreme northwest portion of Tennessee, United States of America, just south of the Kentucky line. 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 baldcypress 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. 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.

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 three days to fill it, but some scientists dispute the validity of this story. 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).

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.

See also: New Madrid Fault Zone