Three Mile Island Nuclear Generating Station is a civilian nuclear power plant located on an island (Three Mile Island) in the Susquehanna River near Harrisburg, Pennsylvania, United States. Its name is derived not from the size of the island, but from the fact that TMI is three miles downriver from Middletown, Pennsylvania. It was built with two pressurized water reactors, TMI-1 and TMI-2. The plant is most well known for being the site of the worst civilian nuclear accident in United States history when in 1979 TMI-2 suffered a partial meltdown. The core of TMI-2 has since been removed from the site.

The plant was originally built by General Public Utilities Corporation, later renamed GPU Inc., and operated by Metropolitan Edison Company (Met-Ed), a GPU subsidiary. Following the accident at TMI-2, the plant was transferred under the ownership and operation of a new subsidiary company, GPU Nuclear (GPUN). GPUN continued to operate TMI-1 until its 1998 sale to AmerGen Energy Corporation, a joint venture of Philadelphia Electric Company Energy, Inc. (PECO Energy) and British Energy Group Plc. PECO's share in AmerGen was acquired by Exelon Corporation in 2000, after that company was formed from the merger of PECO and Unicom Corporation. Exelon acquired British Energy's share in AmerGen in 2003, and transferred the plant under the direct ownership and operation of its Exelon Nuclear business unit.

The damaged and deactivated TMI-2 remained under GPUN ownership until 2001, when GPUN was acquired and absorbed by First Energy Corporation. First Energy continues to own TMI-2, but has subcontracted the maintenance and management of the site to AmerGen Energy from 2001-2003, and Exelon Nuclear from 2003-present.